Serfs had fewer incentives to work more hours.

Capitalism is optimized to produce products and services people want to buy which drives people wanting to work more hours. Which then drives up the costs of things that aren’t replicated endlessly like land and goods that are intentionally left scarce such as admission to elite institutions.

You can work dramatically less and still live well, but the sirens call is very seductive. Food, clothes, transportation, housing, etc all offer real value at minimal cost and little pushing you in that direction.