Because chariots were pretty much a one-trick-pony at this time. Most people had figured out how to beat them, and heck - even if it is a massive wooden vehicle, it still has two horses at the front. Take out one and you've made the vehicle useless. They're still handy - breaking up formations and such - but are generally inferior to heavy cavalry.

So yes, chariots are (were) basically the most pointless form of warfare.