She does give a reason. Her dad lost his job and turned deadbeat, drinking away his issues and treating her like shit, so she fell to prostitution since it is guaranteed easy pay and she has to pay for rent AND her dad's drinking habit.
The game is only really setting her up as a friend at this point and a lot of people are okay with that. Not every hole needs to be conquered.
I mean with their(the people complaining) logic, even if Chika didn't exist the game would still be netorare in some form since Miharu is madly in love with you and you end up fucking other girls. Hell, even that one girl in the student council could give a perfect example of what the player(not the MC, the actual player) is like to the other girls. She wants to fuck you, and does end up fucking you but you end up fucking other people. Oh no, ntr(and that isn't actually ntr based on definition so they're just complaining)! Yet no one complains until they're the ones being cucked. Gooners. Fucking jokes.