As the UH-60 series were designed for the US Army and are ostensibly a land theatre platform, therefore not much of a need for floats, why add weight, cost and maintenance for a rarely needed option.
As for the over water H-60 variants, the crews HUET training is far, far more rigorous than say for commercial SLP, as such, the operators consider the risk/benefit analysis is for a ‘no floats’ requirement.