Mostly because normal breastfeeding is "novel" to this generation/this country.
"Extended" (aka normal) breastfeeding goes to 1.5-2.5 years old BUT some mom's aren't understanding that it's a MIX of foods (solid and milk) during the second half of it.
Now.... 6mo for solids is RIDICULOUSLY early (and is a holdover from back when formula was waaaaaaaaaay dangerous, and you wanted infants off of it as soon as possible)... only ABOUT half (some studies show about 1/3) of infants can digest solids at ALL at 6mo. ((Yet another thing being studied in conjunction with American Obsesity is "starving" infants, by filling them up with tablefood they can't digest so young. This isn't starving as in lethargic / not growing... but creating a deficit in nutrition, which sets their metabolisms very very low so they hoard every calorie that they ingest OR their bodies become incapable of properly putting nutrients where they belong. This study is going to be a long one... so look out for it in about 10 years... but prelim CORRELATIONS show that babies who aren't given table food until at LEAST 1 year tend to grow better and have healthier weights until puberty. THAT study (swedish) is long out, and finds an absolute correlation between dieting and puberty = life long weight struggles (overweight) but that's a different kettle of fish on the same fire.))
Yeah... there's no magic birthday "switch". It's a whole complicated thing. That would take pages and pages to go into involving growth rates (the faster they're growing the LESS table food they should have when breastmilk and formula are available... of course, older kids need to chow down ridiculous amounts of food to support growth spurts), activity levels, digestive maturity, etc.
Which you Ped PROBABLY knows some, but very little, about. Meaning... never take a Ped's last word on nutrition. Take someone with a 4 year degree in nutirion and an extra 5th year in infant/early childhood nutrition over theirs. Which is who they will send you to if there are honest to god problems (or medical need, like disorders, allergies, chemo, etc.)