You can't use the real-world fact that a horse is heavy to justify the insane cavalry mechanics in the game currently, because:
1. The weight of an object moving laterally doesn't (much) affect how hard you get hit. Gravity is a vertical force which doesn't really affect momentum. A 40 ton tank which hits you at 20mph is going to impart just as much force as a person who hits you at that speed. If you're standing in a faultline and a moving tectonic plate taps you at 2mph, you won't suddenly get knocked back because of the mass of the object (which is trillions of times more than a horse). Due to the latent elasticity in all objects there is also a limit to how much energy can be transferred in a hit.
2. Horses don't charge headlong into anything stationary. They're not suicidal and no amount of training will override their survival instincts. The "charge" we often read about in sources is usually also a rout by the infantry before they get hit.
but more relevantly,
3. Mount and blade is a game with a disproportionate amount of heavy cavalry, very small battlefields, and no cohesive formations. Even the biggest battles in warband are more like foraging skirmishes, and it's difficult to balance cavalry to be realistic but also not completely wipe the floor in those situations. The solution is to make it much easier for individuals to kill horses which are charging them head on, hence the spear-rearing mechanic. Without these concessions it would just turn the game into a bull in a china shop, which may be more "realistic" in a very short-sighted sense, but doesn't benefit the game.
The lack of morale isn't the issue, it's the ability of horses to crash through multiple lines of stationary infantry without taking much if any damage, while imparting more damage than some arrows. It's absurd.