Why isn't UserId more readily available

[SoftCircuits]

If I need the current user name, I can use User.Identity.Name. No problem.

However, if I need to save a reference to this user, the user name can change. Better to store the ID instead. But I'm surprised to find that User.Identity.Id is not a thing.

In fact, my research so far suggests the ID is not available unless I get an instance of UserManager, or query the database.

It seems that the user ID is at least as useful as the user name. What is the preferred way to obtain it? And why isn't it more readily available?