What exactly is the point of giving me onagers as a reward or at the start of a campaign scenario if the AI just flawlessly dodges every single shot I take?

This applies to almost every campaign mission. Currently playing barbarossa 4 and the game joyfully grants me access to 5 pristine onagers. Naturally, I try to make use of these units to take out the enemy's beefy cav and archers only to discover it was all in vain. Literally every single shot I take is dodged and an immediate counter attack ensues.

I've tried this scenario a handful of times and at this point I simply delete the onagers, since they're just there to take up pop.

This is an issue in skirmish as well. Say I want to practice a strat that involves attacking with onagers. Nope. I'll be playing versus an opponent that micros every individual unit to perfection, dodging every shot. Every resource spent on these units might as well be thrown into the bin.