Your point of view is understandable. The difference is I see harem games as cheating by having multiple lovers, and if the girls are all okay and happy about the situation, it throws any realism or drama for a good story right out the window. There's no sense of accomplishment if everything is handed to you without some struggle or work involved.
You also forget that this is fantasy world, where powerful and important people are basically expected to have multiple wives, to ensure that their line continues. It's a cultural thing in the world and people in many places grow to even expect it from those in power. MC rising to the position of archmage would constitute him being also in that kind of position, especially with how the world is progressing. Also Talothral is good in writing that there are issues that people in the harem have with it, and with each other, during the story, that are dealt with during the storyline. Also it's not that everything is handed to you if you go for all LIs, there's multiple decisions you have to take to reach the point where multiple LIs are all ok being with you simultaneously.
Not to mention that having multiple wives is beneficial also for the partners in the scenario of the game, they have more people they can safely rely on without having to worry about potential betrayals. There's multiple options for training their own children, since many of the other future wives have different expertises around the field of magic, or other important skills like thievery, so they can guarantee that their kids will receive the best possible education. Also with multiple wives there's always someone around the kids, that has more vested interest in their well being than any hired nanny would have. These are all points of views the LIs themselves point out during the game in their discussions about the issue of having to share MC with others.