Why are cavalry units so slow and weak?

[Vincent Butler]

even when I charge some skirmishers and leave them such little time to react that they're just beginning to change directions to flee, I still get no charge bonus.

I have noticed that too, I have to watch, and if I don't hear their horn, or if the mouse-over description over them does not say "Charging", I will cancel the attack and move back to try again.

but I still think these heavy cavalry units are exaggeratedly slow and tire out too quickly to be useful or worth the insane cost. I'll see what I can do once I'm able to recruit non-mercenary light cavalry.

They tire out very easily, I didn't like that either. By the third charge, even if I hit and pull out, hit and pull out, they are already winded, and that is just Equites Romani, ditto for Generals and Hippeis.