Because you still have a lot of motors which would require inverters if the house supply was DC. Your fridge, your washing machine, your dryer, blower motors, A

Because you still have a lot of motors which would require inverters if the house supply was DC. Your fridge, your washing machine, your dryer, blower motors, A/C compressors, kitchen appliances, all of these require AC and a lot of power. If we switched to DC (little alone low voltage DC) all those things would need their own inverters and increase the cost of them because inverting properly is hard. Compared to DC which can be rectified from AC by four diodes and a capacitor. It's trivial to convert AC to DC and it basically costs pennies.