So what do the girls always like about MC-tan?
They're obviously high-tier and he seems pretty low-rank.
Because it's Touma.
Because he promised that he would marry one of the girls back when they were kids and because one girls wants him the rest flock to take him.
An invocation of the law of 'fuck-you-self-insert-fantasy-sells'
You're not supposed to question it. You're supposed to pretend you are the MC and mindlessly jerk off and buy merch.
Is that Touma?
Reminds me of Issei
Well in Touma's case, he's a batshit insane fucker that to the girls looks like a great hero. He also gives less of a fuck about the harem than other MCs, mostly because he's busy with actually important shit.
He secretly feeds them a special aphrodisiac that makes them gush when they look at them.
Seems fine to me
They all look the same