Because if your billing address doesn't match your current address, it won't let you buy anything, regardless of if you're using a bank card or paypal or whatever.
Considering no other store I've encountered has this issue, I would expect there's something that can be done about it, right?

Yes, I could probably use a VPN, but the moment you start asking customers to use VPNs just to purchase games I'm going to cry for developers who are missing sales when customers decide "eh, not worth it" and/or go and obtain the game through other means. The legitimate way of obtaining a game should always be the most convenient.

Is there some sort of reason it's like this but other vendors don't have this issue? Is it just a pain or it requires third party authenticators? I'd like to know.