Because if I am interested in playing a wholesome romance game with a focus on 1 girl, I want it to be meaningful. If she is capable of sharing me with another woman 1) I don't respect her. 2) I don't feel loved enough by her.Ok, we are greedy. I won't deny, but there are quite a few harem games that give the option of not going after all the characters, so how is playing it like that any worse than having a game force you to only pick one?
If anything it would be better that way because the player makes the choice and isn't forced one way or the other by the story. If multiple endings are planned anyway...then why not?
It is like having an optional NTR path for every girl in a harem game. You don't want to know that they are capable of fucking someone else, even if it is optional.
Makes sense?