Because software is incredibly elastic, which heavily blurs the line on what is and isn't a new invention. It is also nebulous, which means that it's "easy" to

Because software is incredibly elastic, which heavily blurs the line on what is and isn't a new invention. It is also nebulous, which means that it's "easy" to obtain a patent for a system that seems reasonable at first, but then decide "actually I meant all of this other stuff too" and on paper the patent does apply to all the other stuff too, because short of submitting the exact code to be patented you pretty much always get to do this.