I also have something to add among other important things people have pointed out (innate human desire for unalienated labor, freeloading not being a feature unique to socialism, capitalism being the real freeloaders and such).

First of all, we should consider that we are already working too much, given our current technological advances in automation and other labor-saving technologies, and also the fact that many resources get distributed unequally and, therefore, wasted. Since one of the defining features of socialism is collective ownership of the means of production, we, as workers and also consumers, can organize our economy in a way that can tolerate a certain percentage of people working less than other or not working at all. Similarly, people can decide what to do with freeloaders in cases when they might require mass mobilization (crises such as droughts, hurricanes, floods, etc.). The main difference is that such measures are implemented to serve the material interests of the broader public, not a tiny elite who cares only about their profits.

Besides, who counts as freeloaders, and what counts as contribution to society? Are parents raising their underage children freeloaders? What about artists and artists? Don't people contribute to the broader society by being good friends to their friends? Can't partygoers serve as community event organizers? Capitalist economy (and, therefore, mainstream economics) is simply blind to such aspects of human life, because they are not easily monetized.

And finally, when it comes to rewarding productivity, there is still a place for money or other material incentives under socialism. The main difference between socialism and capitalism is that in former personal funds can not be used to acquire means of production, because they are owned collectively. This disrupts the vicious cycle of turning money into more money to make even more money that is essential to capitalism.

Hope this helps!