Socialists, Why not mandate minimum/living wage $50/hr, $100/hr, or $250/hr instead of $15/hr?

I'm curious as to why $15/hr seems to be the agreed upon minimum wage. Socialists, why place a cap on the minimum wage/livable wage? Does it not make more sense to have a higher minimum wage/livable wage as it could, supposedly, do more to fix economic inequality?

EDIT: Thanks for all the replies! I'm merely trying to point out the flaws of trying to provide people with a basic income with no free market to value it against. I understand that, to socialist, the notions of wage slavery want to be abolished, but how is a basic living income any different (except that there is nothing to value it against).