Why don't more homes have at least one UPS (Uninterruptible Power Supply)? Would more people have one if they knew about it?

Don't need one. I only have my laptop running, and play with servers on the cloud (Azure/AWS). If I had servers running, then a UPS would be beneficial.

I suppose that a whole house UPS would be a great idea in theory, but having set-up UPS as part of server farms. The UPS can be very large, heavy and could cost $$$ to replace the gel cells every 3 to 5 years.
Your continuity of operations plan for your house should be driven by requirements. In my house we use only laptops, or tablets.

I have one because I have studio equipment that don't enjoy power outages.