It's because a UPS normally last longer than a power supply.

I live in some dusty area and the power supplys of computers at home usually last no longer than a year even when I regularly remove dust from it. A normal UPS can last for 3-5 years if you care to replace the acid inside every 6-9 months.

And note that the process of "replacing acid" can damage the power supply easily if you're not careful. Common rechargable batteries are good but expensive. I remember that the battery of my IBM notebook cost around HKD 1000 each.

Also, even the "entry level" UPS can serve up to 2 machines at the same time. It's more convenient to build them as seperate parts.