Accidents happen... you cannot predict that.
Sure, the parents will always be in the same room as the baby... but there will be times when you are not. And yes... kids/babies DO wander... and like a magician.
And there are many situations... in which a child moves SO fast... right before your eyes as you glance elsewhere... and then you do not know where they went.
I have "lost" my kids in my own home. And, as a child gets hits certain age stages... they can get very hard to keep track of e.v.e.r.y single second of the day... and I am not a careless Mom myself.
No, it is not that simple.
I say, they are necessary and worth the expense. And yes, as Riley said... pet gates are also a very good option.
As for whether gates are needed only for homes with stairs... I disagree. My friend, who has no stairs in her home... had lost her boy once. Because he had managed to open the knob of a main door... and got out of the house. Yes, scary. Luckily, he did not stray far before she found him.
All the best,
Susan