What is the fundamental purpose of ID?
This is an interesting one. They may or may not be successful in their aim, but even if they are, why would it be useful?
What is the purpose of ID in the context of access to systems? It is to show that a particular instance of a person or system is entitled to access. It's no more than a door key. And if I am authorised to open the door on my house, and also the door on my office and to start my car, there is absolutely no need to link the three together. If I have a key to a particular door/mechanism I get access. "One key to rule them all" is not a great idea.
Same with ID - universal ID is unnecessary, and probably even a bad thing. Why should I use the same ID to buy a pair of socks at we-are-socks.com as I do to view videos on nubile-squirrels.com or submit my tax returns? The government may want me to use the same ID for all my interactions with them, but that's about all. I can see why Google/FB etc want to be able to link my sock purchases to my viewing habits, but that's so they can make money, and I will do my best to screw up their data.
Obviously there are issues with stealing/copying keys/IDs - but given that absolute 100% security is not possible (Newton's 4th Law) the risks are minimised by having different identities/keys for different things, to minimise the risk of wider problems if one ID/key is lost.