All of them? Certainly not. Many? Yes.
>> There might be some friend groups who split the cost of an account between them, but I suspect far more common is that parents have an account that kids (who might be independent adults) use, couples who don’t live together sharing passwords
Yes, but although we don't have details on demographics, in Europe it is widely "accepted" (for lack a better word) that many friends, relative and even acquaintances share an account and its costs. I don't have number so consider it totally anecdotical, but it is honestly common sense, especially in southern and eastern Europe. (I am from Italy, FWIW)
edit: this is also true for Spotify Family accounts. Agains TOS? Sure. Do they do it anyway? Yes.
It is very common sense here to say "Ok, I'll cover Netflix, you'll cover Disney+/Spotify/other"
The moment when one of this services doesn't allow sharing anymore, this system will fall, and users will be incredibly more selective to what they will subscribe to.
I suspect this might be cultural, and not just at a country level. I wouldn't dream of charging a family member £5 a month for something, I'd either let them use it or not. But that may not be true everywhere. Between friends it makes a lot more sense, but as I said I've typically seen Netflix/etc shared in families not friends, probably because the lack of money changing hands makes it easier.
There might be some friend groups who split the cost of an account between them, but I suspect far more common is that parents have an account that kids (who might be independent adults) use, couples who don’t live together sharing passwords, or more well off siblings letting less well off siblings use their account.