Because 100$ per hour is not what labor is currently worth.

The whole idea behind making the minimum wage a livable wage is to stop allowing corporations to keep wages artificially low by having workers augment their wages with government provided subsidies.

So what people are saying is, when you account for the subsidies people currently being paid minimum wage are taking advantage of, the real cost of that labor is much closer to 15$ per hour than the current 7.50$ per hour.

Shifting the minimum wage to 15$ an hour will shift many people with minimum wage jobs off of government subsidy programs, actualizing the real cost of labor on corporations and lowering the burden of our current government programs.

so this is much more than just picking a number and saying 'wages should be at least this high", its more "youre are paying X, we are subsidizing that by Y, we think you should pay all of X+Y"