You overestimate the power of a bodkin arrow, or a bow for that instance.
If you hit someone in the chest they will for example not even have a bruise, because at the time the arrow might
potentially hit a padded cloth the arrow's metal piece is blunted down so much that the plate armor's wearer won't even get a bruise.
I can imagine minor disorientation, but yeah thats about it. A longbow within in its killing zone(=20 meters) can't penetrate a chest piece enough to harm its wearer. In the test it didn't even harm the padded cloth below the chest piece.
Furthermore I don't expect a single person to hit someones head with any arrow 5 times in a row - baron already mentioned some of the reasons earlier, the other one would be that those arrows would likely already stick out of a shield before they get to hit the head.
As Gregor already said earlier, a baseball isn't the same as an arrow tip.
First of all, the baseball won't get deformed when hitting the players helmet while the arrow tip most certainly will - that's the point of the plate armor.
Secondly , a baseball is generally a bigger spherical, blunt object while an arrow is a thin pointy object.
As @
GregorWaldstein said, I'd also like to see your source for this as it is far more likely that said longbow hit the horse than the Knight. - Its not that easy to throw someone off a horse if they really want to stay on it you know.
Especially not if the Armor had that nice addition between the legs (yeah seriously; haha that thing between the legs haha very funny - some armors had that look it up) you could mount on the saddle.
Next thing is the likelihood of someone even hitting a moving object with a bow as it has a fixed trajectory.
People can avoid getting hit by bullets and paintball bullets (believe me I tried the latter myself and I didn't get hit even though they had OP painball guns that were
barely allowed on that field)
Hello, my name is Nobody.
(Kudos to those getting the reference)