Marx begs to differ. By labor theory of value Sisyphus should be the wealthiest man on Earth; unless you smuggle all the complexity and paradoxes of this theory into some ill-defined notion of "socially useful labour" (how does one measure or quantify utility?) of course...
Don't mistake "it takes a lot of effort" with "it is hard"
Digging holes isn't hard, it just takes a lot of effort