I can’t speak for Asana and explain why they haven’t done this. But I will say that user access control can get complicated quickly. And the more feature rich (AKA complicated) you make it the more rope you give the users.
For the freelancers, they have My Tasks to see all Tasks assigned to them. And can use the search Tasks and Conversations I’m Following for all Tasks they are following. Not perfect for what you want, but functional.
For you, you have full view of all the Projects and Tasks.
There is an existing topic in Feature Requests regarding user permissions you may want to check out.