The Linux kernel has probably dozens of actively developed forks.

Do they usually take upstream changes from the main kernel? If so, then they aren't the kind of forks we're discussing.

Think of a different scenario: Linus does something that pisses a lot of people off. (Even more than usual) The biggest contributors decide they don't want him in charge. The users don't want anything to do with him. A few of the most influential contributors announce a fork, some of the fans publicize it, and then the major distros base themselves off of the new fork.

At this point none of the forks that you're referring to take upstream changes from Linux, instead they take them from the new fork.