The problem is that multiplayer games and any game with a server component - and the engines that make them - are specifically designed to have security against cheating and hacks. No-one wants to play against a modded game. That makes supporting mods, even solo mods, all the more difficult, and it’s harder to make the business case for doing so.
Au contraire. There are plenty of people who would like to play against a modded game. They just might not want to play against a game that's modded differently from their own.
Yes, I've recently been playing Fallout: London, which is a total conversion mod for Fallout 4. It obviously changes the setting to post-apocalyptic London, but also adds new quests, monsters and things. And it's pretty good quality -- in fact the mod kept getting delayed because Bethesda saw the mod in progress and kept hiring the people working on it because they were so impressed.
But yeah, they're few and far between.
There are lots of games that will let you mod the ever loving crap out of them. UEFN and Roblox are modding with distribution platforms. Cyberpunk has Bethesda level modding. Games like lethal company are existing in an indie space that was just unfathomable when I was playing those games.