All laptops have a built-in UPS, and in fact one that generally can last at least a couple hours.
Why don’t all desktops have a built-in UPS? It wouldn’t have to be any bigger than the one that would be in a laptop. Usually, for the same cost a desktop gives you more features than a laptop, and it’s just silly in this day and age for a power failure to take out unsaved work on a computer.
I suspect its just a cost-reduction measure — if you want a UPS, its assumed you’ll buy one separately. As for a UPS running a desktop not lasting as long as a battery powering a laptop, well, that has to do with the internal architecture of a laptop. Laptops tend to be very power-conscious machines, and do a great deal more of turning things off when they’re not in use to conserve power — for instance, my video card shuts down its more extravagant features when I’m not on external power, and it produces a noticeable performance hit. Desktops are designed for a different mission, one in which they can assume to always have power available, and those choices are reflected both in their standard equipment as well as in their construction choices.
Desktops use far more power than laptops, so the same-size batteries would give a small fraction of the runtime. Another problem is that batteries can only discharge so quickly, so you cannot turn a 15-minute UPS into a 1-minute UPS by shrinking the batteries 15-fold.
Well, if the one in the laptop can handle it for a few hours, it seems that the one in the desktop could last for 10 minutes. Really, I’d just want it to condition the power and be able to shut down a computer “cleanly” when power failure occurs.