I asked a mod to delete a post I just made. Instead, it was just locked, but it’s still visible. Any reason why?
Without seeing the post itself I can’t comment on the merits, but locking and leaving is the usual course of action. Mostly what you say here stays here.
We do ask that you think twice before you hit the button once. 
your humble TubaDiva
**elmwood ** - Short technical answer is that it thoroughly and brutally thrashes the system to delete a message and re-set seventy-kebillion pointers in the underlying database to make it disappear.
It’s far less harmful to the hamsters to simply make it read-only.
Unless you’re a sock or a troll, threads won’t be deleted.