Why do they cast grown adults as teenagers in movies?

I just rewatched The Duff, which came out in 2015. It stars Mae Whitman who starred in a show I just finished, Good Girls.

At the time she was 27 years old playing a 17 year old 💀

In the show Never Have I Ever, they have a 30 year old man playing a high school senior (Paxton).

I noticed this is pretty common, but why not cast actual teens?

That definitely had teen me confused. Seeing grown men with a good physique and I got was some lanky turd lol but in reality that’s just how teen boys look.