Only full nodes need to verify everything. Not every user needs to run a full node.
Why do so many people equate users with full nodes?
b/c you will not know if last 5(or 100) full nodes are cheating. ... you will have to trust them blindly like a sheep. :-)
It is easy to do not verify if you know that everybody can verify.
but .... if only "few" people have resources to verify ... and even less will spend(risk) resources to verify then who knows if blockchain is still valid ?
If you are pool miner who only create hashes and is not verifying blocks then cartel of pool owner can create bitcoins out of thin air. (and nobody will notice until it is late)