"If you work harder you will be more successful" is the fundamental axiom of American conservatism. Of course I can't speak for everyone, but the primary reason to oppose a social safety net is "those people are poor because they aren't working hard enough." This requires a belief that hard(er) work is and should be rewarded better.

I'd say your claim is nonobvious. That the world is fundamentally fair, and that hard work is the most important aspect of virtue, are deep ideas in American culture and very much tied up with the American Dream (TM).

Contemporary liberals typically challenge the "world is fundamentally fair" part, appealing to structural *-ism that makes this untrue for people other than white men born to financially secure parents.

Few (economists, maybe) would argue that how hard you work does not influence how much you deserve.