aah yes but English Longbowmen could easily fire 6 arrows a minute so if there were 300 longbowmen in a unit they could fire 1800 a minute so the cav would either be dead already or they would flee, unlikely as it is IF they did carry on charging they would be so few that they would be picked off.
and then? they had no aimbot in XII century. They usually thrown tens of thousands arrows in battles, but they could not kill so many people, due to low accuracy and enemy armors.
Try to hit a horse charging you at 15 m/s. It take 20 seconds to reach you and even if you are the fastest archer in the world, i don't really think you will be able to throw more than 2 aimed arrows.
"horse are bigger and easier to hit" is pointless. An air plane is not easier to catch than a mosquito just because is bigger. But an air plane is not easier to catch than a mosquito just because is faster. It's depend on what are you using: a rocket can easily strike down a jet, but can't even track a mosquito.
Archers sucks in melee 'cause they lacked of armor, armament and training. And what's the fastest way to reach them and start a melee fight? A man with 50+kg armor or a light and fast horse? As i said before, heavy infantry could not even reach archers.