QE increases the supply of money, which reduces its value, just like most things that are in demand have their price lowered when supply increases.

The reduction in value of cash transfers wealth from people who are cash-rich to people who are asset-rich or have large currency-denominated debt. And because things like salaries are sticky, it also devalues salaried work; this is only partly offset by stickiness in prices, because some goods are imported.