Seriously, what the hell.

Here's what happens...

They run after routers and stop as soon as they reach them.

They run at the same speed as the routers and never catch them.

Or they run in front, behind and on the flanks of the routers without moving for the kill.

If there's a single router left it's even worse. You knights will run in all directions, regroup, scatter, run toward the router without attacking... Watching this go on at 3x speed for 5 minutes is quite ridiculous.

The only option is to "trick" your cavalry into killing routers by making them move among the routing unit first and then giving an attack order. This is obviously a pain to do when you have other things to worry about on the battlefield.

Why can't the game do it? What is so complicated? Is there an evil programmer at CA who secretly implented a script to make cavalry avoid routers at all cost? Because that's exactly how it looks in game.