> Do you know why Apple or Google doesn't provide this kind of unique id ?

This picture explains

https://imgur.com/a/XYTNRN3

Chinese can recycle real iPhone/Android devices at minimal cost. Anyone can rent a fleet of real devices, then RCE software can execute any kind of task you want on a real app on a real phone. So even Apple or Google provide some kind of unique id, e.g. iOS already have something like identifierForVendor, the spammers emulate a real user's app download, registration, login process, thus obtaining a real ID. So what can you do about the ID?

So the obvious solution is to check for more user information beyond a simple ID. Your IP, mac address, wifi router address, other process the OS is running, device parameters, etc. and privacy is f??ked in the process.

They have (had?) uniqueIds. But they don't solve the problem of identifying real users. You can simple fake them or take a real one from somewhere. Information-Density of a single value is far to thin to be a reliable indication on whether someone is a real human or just a script.