Question was not "why does the game need a port", it was "why people need a port". Most of nsfw games don't need good PC, so "phones can handle way heavier games" definitely not it.
I think reason is that many people who play these games are actually minors, or at least aren't living alone. If you're afraid of being caught, then safest time for this is when you're in bed and everyone at home thinks you're sleeping. Even if someone walks in you can quickly hide your phone and pretend sleeping.