So many people seem to forget that we live under capitalism. Why is it always for customers and rarely employee compensation? The latter costs money and the former increases already obscene profits. It's capitalism. That's why.
From the employee side, they would get paid more, could pick up shifts at varying businesses based on their schedules. Then maybe at scale, such a platform would be incredibly useful for organizing labor. This obviously would terrify employers.
I'm not really debating, more of just sharing random ideas and complaining online, because this is basic game theory as to why employers wouldn't want such a tool (potentially higher labor costs short/long term, increased labor mobility, unionization risk, other issues like training/liability/insurance/taxes/etc.), but employees/labor could be greatly served by such a platform.