Expiry does more work than revocation
Revocation assumes somebody tells you. Nobody tells you.
Revocation lists are built on an assumption that does not survive contact with the world: that when a licence is suspended, the systems relying on it will be informed. They will not. There is no notification channel, no subscription, and usually no obligation.
A claim that expires needs no such channel. It fails closed on its own, at a date fixed in advance by the person who signed it, whether or not anyone was watching. That property is worth more than a list that is only as current as its last update.
The cost falls on the professional, who has to renew. Twelve months is the cap in RVW-1 and six is encouraged, which is two signatures a year.
Withdrawal still exists for the cases expiry cannot cover: a rotated key, a licence that ended early, a mistake. It is in RVW-6, and how fast it should propagate is open problem OP-7.