This is very close to encroaching on a philosophical debate lol. However, I'll just assume that you're asking rather or not I prefer a single love interest vs multiple of them (even tho I believe you can easily have a romantic harem game. Look at Cozy Cafe as an example.)
I do prefer harem games. They generally have two main aspects, the characters (harem) and the scenario (story, setting etc.). In a harem game, these two aspects compliment one another and the developer can use one to elevate the other. Plus, if the player doesn't care about the scenario, the characters can make up for it. If the player doesn't like a character, then there's always another that they might like. Ofc, there's always the chance that the player may dislike one of the two aspects and thus ignore the other one (I've been guilty of that too), but the fact of the matter is that the choice is there.
For a single LI, the developers primary focus must be on that LI. The scenario only exists to elevate the character, but the reverse isn't as important. If the player doesn't like that LI, the game is dead to them. There is no fallback. There are no other characters to enjoy, and since the scenario only exists for that character, it becomes meaningless. The amount of people who would stick to a single LI game for the scenario despite not liking the LI is the extreme minority.