DKIM selectors aren't UUIDs. You can of course use a UUID as a selector, but you don't have to. My selectors are named S-YYYYMM (when I rotate the keys), so my current public key is at S-202001._domainkey.example.com.
A lot of tools generate UUIDs for the selector, just to get something unique without having to ask the user for something relevant or defining some other heuristic. For instance: the built-in helper tool for Zimbra generates a UUID by default, unless you provide something specific. I think a lot of people assume it should be a UUID just because they see UUIDs used in common examples.
Few people think about key cycling for DKIM as it isn't a built-in requirement at all, so once a UUID is set they just keep it until some point in the future that may never happen when they need to revoke the key because the private half is compromised.
