Because of child labor laws and kids having to go to school a certain number of hours a day.
A young looking 19- or 20-year-old can work a full 14-hour day of filming.
This 100%.
Much easier to use 20-year olds to play 15-year olds than to actually get a 15-year old.
However....it REALLY depends on the show. Sitcoms like Cosby Show, Blackish, Roseanne etc., used actual kids and teens. Stranger Things on Netflix used teens as well.
Then you have Fresh Prince where Will Smith was in his mid-20s playing a 15/16 year old.
Some shows want to display year over year physical/emotional etc., growth while other shows want the "kids" to remain consistent from season to season.
Remember that Fresh Off the Boat show with the Asian family. The Asian kid playing Eddie probably didn't even come up to Constance Wu's shoulders at the start. After 4 years, dude was towering over the whole damn cast...dude must have been pushing 6'2" in the final season when he was probably not even 5'0" in season one.