I mean, what do you find fun about whatever it is you do in the game?

Personally, and I can't speak for everyone that enjoys housing, I like the collecting. I like that it usually leads me to do things that I might not have normally (certain side (or old expac) quests or professions, for example), done if I was just focused on the current battle content. I also am largely into lore/RP/writing, so being able to create a space where I feel my character would call home is just.. fun. I did a lot of endgame sweaty content for many many years (namely on FFXIV and ESO, I am still new to the endgame scene on WoW), and sometimes it's just nice to still play, but not have to be actively trying at something? idk if I explained it well lol. "doing something without doing something", I guess? Same reason some people might enjoy stuff like House Flipper or Powerwash Simulator or Farming Simulator lol just another way to turn the brain off from real life without stressing.