Besides, mounted knights were so heavily armored that if they were to become, say, unmounted, it would lead to them being almost unable to stand, walk, or move. There's a reason why only the strongest breeds of horses were warhorses.
If this hasn't already been commented on, this is
so mythbusted. (Generically, that is. I'm not sure if
the Mythbusters ever touched it.)
Some interesting videos out there of guys in genuine (or at least genuinely authentic) armour running, leaping, doing tuck-n-rolls and then springing back up, etc...
Which is not to say that the warhorses aren't strong, but dismounted knights were by no means sitting ducks. Their problems arise due to the rise of armour-piercing/interfering weaponry (bodkin-ends for arrows, modified farm tools that could be used from well outside of a sword's-reach, etc).
And to cover another point: The big problem with a mob attack against a small number of enemy is when there's no-one motivated enough to actually get to the front and run the very real risk of being chewed up by the smaller force and being the ones essentially 'sacrificed' so that the rest of the mob can close in and take advantage of how the defenders' weapons are now busy/stuck/lost. (Having played the part of a pikeman in a battle reconstruction situation, I know that part of the motivation might be the sheer pressure from the not-quite-so-exposed guys behind you. In this regard, the corridor scenario mentioned at some point would be
worse for the small defending force than a decent back-to-back-to-back scenario in open ground against a much more radially-dispersed set of attackers of a similar magnitude.)