To add a bit to the other answers: Imagine if the protocol required that timestamps increase. Now imagine somebody mines a block with a timestamp a minute in the future as far as you can tell. What do you do? If you try to mine blocks with the timestamp you currently believe is correct, your blocks will get rejected (since they'd have a timestamp earlier than the last accepted block).
Due to the requirement that the network easily agree on whether a block is valid or not, the protocol can't require highly-accurate timestamps as a condition of accepting a block as valid. As a result, requiring monotonic timestamps would likely make things worse rather than better.