Because they no longer have humans policing the site but BOTs. The F4F scheme is just that and it violates Medium Rules.

https://policy.medium.com/medium-rules-30e5502c4eb4

Personally, I shot links to Medium so that they could review accounts of schemers.

If you cheat and take short cuts to get followers there is no credibility. Who's to say "writers" won't cheat by "borrowing" titles, quotes, etc.

There are no shortcuts. Do the work.