perhaps because when it comes to battery backups there are two huge hurdles to overcome.
wattage... for desktop computers anything below around 780w or so really is not all that practical since the time period it would last is not enough to shut off the computer and save your work. if it is too low such as under the maximum draw of your pc it may fail completely and black out your computer.
cost... higher wattage solutions which work as a good failsafe for todays high power computers are not cheap. a 780w model from a major brand... lets say apc is going to cost betwen $150-200. while people seem eager to spend money for a computer they often balk at the prices of battery backup solutions which actually work for them.
just my 2c