Exactly. It shouldn't be.

But, the reason is classism and racism.

The servant is beneath the owner class. That's what it boils down to. Since, here in the US for sure (but not exclusively), black people in particular have been that servant class along with women, of course the owner class wants to pay them nothing.

Since it serves the same interests to have the working class kept in their place as well, shifting the obligation on them means that a majority of the "servants" will be pissed at their fellow working class neighbors instead of the ruling class. It's a win-win. So the laws were shaped to keep that in place, while pretending to help by establishing a bare minimum.

It's bullshit, and always has been, though it was an improvement over having no minimum wage at all