Because the game is complicated enough as it is, with the updates and all. Even if they changed content add-ons to be micro-transactions (which would be a nightmare), it would be complicated to play with someone who doesn't have the content you bought... etc. And that's just for objects. What if you had to purchase worlds or access to a lot where there's an event. There would be little to no people there because not everyone bought it. That's how I imagine it in 'the sims' at least. I even think that if they added the option to build with a friend or collaborate, it would get old after a while and you'd just want to play alone afterward.
