.

Knights have repeatedly fallen victim to simple and straightforward partisan tactics.

There are many knights.

Knights are just one-tenth of the army. Their main purpose is field battles, wall against wall. What use is partisan tactics to them if they won't even enter a forest? Let other types of troops handle that. Lighter, more maneuverable, and not so narrowly specialized.

It's easy! The armor is too heavy; the horse can't handle it. And even in medium armor, it loses most of its already minimal maneuverability due to the knight.

There is a breed of horse called a destrier. 800-1000 kg. It doesn't jump. It gets tired quickly. It is used only in battle, as is combat armor.
The horse is tall; an infantryman can't easily reach it with a sword – and even if they could, a wall of such horses will easily break any formation not equipped with polearms.