Tax implications aside, and yes I understand its a big part of the reason why it is the way it is.
But I think there is certainly some type of inherited institutional bias going on too. In the UK and Australia where most privates are basically open to outside play, I can't even count the number of comments I've read over the years from those members who love their model because it allows them to be a member where they still get all the coveted Thurs-Sunday tee times, and most important all for a tiny fraction of per round cost compared to the US.
IIRC, there are at least 20 courses in the Golf Mag top 100 list from those countries, and I'm guessing most of them are open.