It is not. The idea of higher cav cost is that it lives longer than other troops on average, and does more impact during it's life, so it should have less spawns. All classes supposed to be balanced and equally userful. Having 300hp armored horse usually prevents you from dying in situations other classes have no chance to survive, and it has no downsides. Every class can die from pila or axe headshot and riders are not an exception. The thing that you are only getting dismounted from a direct throwing axe hit, and not getting killed, already gives you a strong advantage over other classes. Also you have a huge advantage of choosing your fighting area, for example if you see an enemy trying to throw franceska at your horse, you have an option to not get close to that guy so he would most likely miss his throw. You can do something else meanwhile.Have you played Bannerlord recently? Horses literally die with one axe throw and spears became very dangerous. Furthermore don't forget that its the most expensive troop, its supposed to be slightly stronger.
If you are dying on cav alot, that's all your mistakes. Good cav players are always have the most impact in the game and they are not complaining about bad game balance, huge throwing damage or unkillable archers. Some things can be annoying as a natural counter, but it is as it should be, every class, even cav, should have downsides