Because I want total control over my game, down to whatever mods I choose to install, how my sims interact with the world, what sims are in my world ... everything.
When I played World of Warcraft, the only time I ever played with someone else was when I was playing with my roommate, and we were in the same room. Otherwise I just pretended other people didn't exist. Same for other online games. I just have no interest at all in playing any game with the general public, or even with friends, really. I tried playing Red Dead Redemption online and RDR2, and found most of the player base seemed to be just out to shoot anyone who hoved into their field of view, whereas I wanted to pretend to be an old west person having old west adventures.
Griefing in an online Sims game would be less lethal, but no less annoying.
They already did The Sims Online back in 2002 and the players ruined it by shaking down new players for money and running online brothels. Then The Sims 4 was stripped down to almost no content when it launched because they wanted to try to make it an online game again. If they're trying to do it again, it's probably because they think they can get people to buy into more microtransactions and drive more mandatory DLC. Right now, if I don't want to get an expansion pack, it doesn't affect my existing game and at most just means I might not have all the stuff that some creators on the Gallery will use. If I don't get an expansion pack in the online game, entire parts of the game could be closed off.
