Basically describing 80% of "yaoi" plots

Why do authors decide to make their characters to be over 20 when they can't write the character to act like an actual full grown adult? Those uni bls with the immature, jumpy and blushy mc over the toxic love interest is one example. Like if you're gonna go with that trope you might as well make it a high school romance. It's more reasonable for a teenager to act so fucking stupid instead of a grown ass man who has already gone through military service