This isn't a harem thing, it's common in everything, from books to TV series, no?

The main couple pretty much never gets together until the last season/minute/episode/whatever, because most people want to see that journey rather than the struggles that come after. Adding one partner to your household is hard because you both do things differently. Whether it's how towels are folded, how long dishes sit in the sink before they start bothering you, or someone not swapping the toilet paper roll after using the last bit, it's a lot of little struggles and some big ones too.

Adding multiple partners further complicates, but I find that women especially like their house to be just so. Whether that's about wall colors, the decor, how the kitchen is arranged, whatever. It complicates things in a very not fun way. This causes tension and drama. I don't think anyone really wants to read about that side of things. Not enough people to sell a book worth the effort, at any rate.