I think the sheer power of knights plus people just “know” them makes them just the natural to use unit.
It doesn’t matter if you have a “better knight” available from a castle, cause knights as they are are already just “strong enough” and you aren’t in danger to lose your whole production because you fail to spot a treb push anywhere.
I totally understand that players step away from that kind of unpredictable risk. The game is about avouding unnecessary risks, so why you should go for that risk if the benefit is negligible?
But the cataphract just doesn’t fit into that category. The cata isn’t a knight replacement.