Follow lists are kept as dead simple as possible, so that it can be reasonably expected for all clients to implement them. We already have a lot of issues with follow lists getting nuked because a client can't find the user's most recent follow list, so it just generates a new one. Imagine the insanity we would have with some clients respecting encrypted payloads in the content field while other clients don't, and just dump the information whenever you follow someone new from their client...