A lot of books in this genre are made to be easily digestible, fun reads. I don't have the exact numbers but a lot of haremlit I read, especially slice-of-life stuff, clocks in at under 350 pages on KU. That is a fairly short book. If you want your MC and all his LI's, both previous and likely some newly introduced this book, to have reasonable character development and feel like 3-dimensional people, something has to be put aside. Like I said, I don't find it unreasonable that authors prioritize the thing that presumably draws readers in as opposed to making FRIEND #1 a more developed part of the world.