Reviewing every post before approval would sure be a lot of work for the moderators and worst still it won't solve the problem.

Irrelevant posts is just part of the issue, and usually gets deleted. The problem is posts which are relevant (or to an an extent) but is flooded with repetitive replies and it goes on and on till it becomes a spam mega thread and thread makers barely cone back to check up on their threads and lock it if necessary.
Mods can not really go through all replies and spam it seems can not be completely eradicate. We can only reduce it with the combined effort of all users.