That's a contingent fact about the place and era you live in. Medieval peasants - the majority of people who have ever lived - were not dumbasses, not all of them - but there simply wasn't a way for even the smartest to accumulate long-term wealth. At best you could maybe get your neighbours to owe you a few more favours, and maybe once in a generation if you played every card right there might be a chance for a patriarch to acquire one more piece of land, but that's it, that's your lot. (Sure you can work your ass off and produce a bit of extra grain in a given year, but then what? It's going to rot, and selling it for money is surprisingly useless to you)