Why aren't all corporation right now paying the minimum wage and giving out the minimum legally mandated benefits to all their employees?
Oh right, because there's a labour market. In which supply and demand meet. The lower the wage becomes, the more there is demand for the labour, the higher it becomes, the more there is supply for that particular labour. This means that the corporation can't "crush people under their heels", as there are always other competitors who are willing to hire people if the price is right.
It's not the corporations fault if you can't take a day off because you have too low income. Your labour is simply not worth a lot in that case.