If it worked that way, I'd support it completely! I think the idea is great. I just don't think it can be implemented in a cross-instance way, short of having some sort of central server.

With the feature the way it is today, it makes it a little easier to follow users on mastodon.social, and a little harder to follow users anywhere else. Suppose that hypothetical social.example.com instance has 1 user. If that feature's enabled on the server, anyone wanting to follow that 1 person has to complete that extra step with zero benefit. It's only useful for larger instances, benefitting the few centralized servers without helping the federation as a whole.