Do you play GOTM? Knights are the bread an butter over most of the games, cavalry gets used when its possible but it is often not possible.
Cavalry if you can get it early enough is great but is much harder to get than knights and also is outscaled much earlier (35 Vs 48). This out-scaling difference and the sacrifices to get to horse early enough are to some degree a challenge, sure Cavalry are great when you get to them but it is getting there that can leave a large gap. The 5 MP is also great and so horses are a good option, just as I said harder to get and you make sacrifices getting there (like do you have enough time to get three archers out).
You could say knights are the fallback as long as there is iron but they are by no means a weak fallback, its more about the speed of getting to them.
Of course with Gilgamesh knights are a no-brainer.