This "reciprocal" social contract is just a myth propagated by the feudal lords then and over romanticization of the medieval period since then. It never really existed.

Who do you think the lord actually sent to do the protecting? Knights? Very few and far between and most were lords themselves. No, the vast majority of armys at the time were made up of pikeman. And why were there so many pikeman? Because you could make them cheap and put a pike in an untrained serfs hands and march him off to battle and he would be pretty effective, at least in numbers. So see the ones actually doing the protecting were the serfs.