The client does check, but it may not have the up-to-date info on all the entities. Only the server can say for sure that "no entity is occupying this block's space at this tick".
The server should send an update when the placement is cancelled, have a look at the hook documentation in the wiki (hooks should be up to date there).
The server should send an update when the placement is cancelled, have a look at the hook documentation in the wiki (hooks should be up to date there).