Because maybe we shouldn't treat human beings solely as economic goods? Sure, the market exists, but it's not like some giant metaphysical thing with laws that bind us the way that the laws of physics bind us. It's okay to do the thing that's more "inefficient" if it better promotes human welfare
But this assumption does not match our understanding of the effects of market manipulation. Well meaning minimum wage laws, just like price floors, can create ineficiencies that are net detrimental to to society.
Imagine an extreme scenario where implementing a minimum wage gives a raise to 50% of the population, while 50% are laid off. I feel like this side is too often neglected in the minimum wage debate.
I would argue that it is better to have 50% of the population on unemployment payments rather than having them employed under below-human-dignity wages. The later looks better on the statistics because those people are working.
The extreme scenario you propose is in my eyes an acceptable result of a minimum wage system, because implementing a minimum wage says "nobody should work and earn less than X$/h". For people unable to find work under these conditions we have social safety nets (well, the US doesn't but other countries do)
