My personal opinion? When I was a little kid (and my younger brother too), parents didn't bother coming in unless they wanted to drink a margarita or something.....BUT the big difference then was all the families knew each other already. We lived in a neighborhood where it seemed most the parents were approximately the same age, with the stereotypical "2.5" kids, everyone played at the same park or community pool, my friends were on my sports teams and all their parents went to the games and knew my parents. When families know each other, it's easier to drop them off. Today, I make a specific effort to at least introduce myself to my son's friends, because it's different. Not knocking people too bad, but when I go to my son's kung fu classes, 3 to 5 parents show up in a full class of 20 kids, and all but me and ONE dad actually watch the class and pay attention, and talk to each other some. Everyone else has their noses firmly planted in their iphone or ipad and never look up. In soccer, it's the same. I volunteered and was a team leader / chaperone yesterday at my son's field trip. 5 classes of 25 kindergartners in each class on a trip, and there were teachers, assistants, and FOUR parents that showed up. So we don't know each other the way my parents knew my friends' parents (even though they didn't necessarily hang out together, they knew each other). I think that makes a big difference in the "why" and "how" this trend came about.
I know that I didn't have parties with children except a neighbor girl and my cousins when I was very young, nor did I get invited to them......the very young magician and bounce house type stuff where 20 kids show up is fairly new to me too: in the past 10-12 years, I'd say. Perhaps because more parents have to work and more children are in daycare so they know more people earlier? Usually, the parties with lots of kids that weren't related or "screaming distance" were reserved for school aged children.
For us: we personally don't send our children to someone's home without knowing them at least a bit. For the couple parties that my son was invited to (age 4, daycare stuff) where we went, the invitation was for a birthday and bbq. That made it adult-friendly. I phoned the mom to RSVP, ask for ideas on gifts (since I didn't know the child), and she said "You and your husband are welcome to come". I brought a gift and a side dish, we had fun too. The host parents facilitated the party, with the parents' permission I was the "designated photographer" and emailed them the snapfish.com link for that album, we parents hung out and got to know each other while the kids played. It wasn't a big deal.
Other parties we've attended as a family, they were neighbors and family friends. The men hung out around the grill and drank soda or beer and talked "guy stuff", the moms were spread out around the kitchen and living room talking and playing OR helping facilitate the party, the children were outside in the backyard or playing games, and it was supposed to be like that because we were ALL friends with EACH OTHER.
For my own children: my little guy who's 2---we didn't have real parties for him yet-just family stuff, but we've invited my friend who has a 2 year old to come play at our favorite place and then had a picnic. Last year my 5 year old's party was tons of fun. I asked that same friend with the 2 year old to stay and help because she's my FRIEND, and her 2 and 5 year old boys are friends with my 2 and 5 year olds. She's the only one I asked to stay, but during RSVPs, I let the other parents know they were welcome to drop off their children and do some shopping or whatever, OR stay, it was their choice. 2 parents originally planned to leave, but ended up having fun and stayed for the whole thing. My husband made 2 friends that evening, and still does guy nights with them but he met them at that party. The neighbor was just about to leave when our surprise guest, Darth Vadar, came in....and she ended up staying too. She found it awesome to watch her son's reaction to Vadar just walking up like that (they had to fight him off), and I think that with parents working a lot and there being a ton of scheduled activities for so many on weekends, sometimes the parents just want to watch their kids in their "native habitat" and sit back and watch how they act. Time is short, it makes sense to me, I won't complain. I always make enough food for all siblings and parents. If we have leftovers, that's ok. I don't expect that parents will keep coming (especially if we don't know them well) now that my son is about to be 6, but I don't mind them. A party is fun for all as far as I'm concerned. We haven't had a party where we had to pay for adults/chaperones. Except once: a pirate cruise. But that wasn't a birthday party, and we just paid for the 2 parents we invited. :P
We did send my 5 year old to a Christmas / Polar Express party where they came in pjs, made cookies, ornaments, played, then got in their sleeping bags to watch Polar Express in the living room. My friend was hosting the party and was like "this is a big kid only party, and pick up is 10pm" and I said "Awesome", and that was it. We put the youngest down to bed and had a date night at home, lol.
I know this is a LONG answer, but just wanted to answer your question as best I could: for us, it's not a helicopter parent thing but a social thing. We don't invite people we don't like, I suppose? We won't invite an entire class when my son is only friends with a few of them. We go to parties because we're familiar with them or want to get to know them....if we don't know someone at all, or don't WANT to know them better, then we aren't going to their party either. No harm, no foul. And yeah, kinda recent. Not overnight new, but a shift in society: less parents know each other personally, more children are involved in more things (daycare, preschool, a million activities, the trend that some people think they have to follow in inviting the whole class instead of just the ones you want), more parties at younger ages, less family time so some parents want to be around to watch their children play in a less structured environment, or some parents actually being really good friends or wanting parents with the same age children and it's a social event for the whole family since we go to bigger churches but no less people, lots of events but everyone is stuck playing on their little gadgets, etc, etc. People are social creatures and I think the evolution makes sense. I won't be bothering to go to a bunch of parties now that my child is school-aged unless it's a family event (you get that idea while rsvp'ing, which is also looking like a lost art) or I want to know the family. But again, the parties I give are for everyone (open, optional to leave or stay but I do request an rsvp and will contact you if I don't get one in a timely fashion), but we keep the invitations down to actual friends (or someone we think needs a friend), not tons of kids I can't handle on my own.