Why?

Why do authors STRUGGLE to write adult characters? This is a romance, not just BL, trope I’ve noticed. Where the only way the author knows how to write the adult MC falling for the ML (if MC is uke or woman) is by writing them with the mentality and critical thinking skills of a particularly dumb 15 year old. Like they’ll be 25+ years old but reacting to the ML’s advances, or any situation in general really, like they’re just hitting puberty. It’s so frustrating.