Why UPS are not DC output?
The obvious thing that you are missing is that many devices will be destroyed if you apply a DC power supply to them. 

Most older electronic devices use a power transformer to convert the incoming voltage to the levels required by the device. Applying DC to such a device will damage or destroy it. 

Most consumers don't know if their devices can accept a DC supply. And because the power connection is the same for all of those devices, it would be not unreasonable for a consumer to plug their device into the UPS. 

Can you say  "Class Action Lawsuit"?