A well designed 2.1 PC setup (used as intended) can definitely sound better that a set of much more expensive, but poorly designed speaker. Unfortunately, Polk is a "hit or miss" company - some of their speakers are pretty good, but others are crap!
As for the 2.1 PC speakers, they save money by not being able to play very loud. Since the idea is you would have them on your desktop and you would be sitting fairly closely, inexpensive drivers can be used and the cabinet does not have to be as well braced because they are smaller and will not see the volume levels required to set off resonances!
As a more extreme example, a $20 pair of Sennheiser headphones will easily compete with most $400/pr speakers! Only having to produce quality sound within the "air pocket" of the headphone eliminates so many of the problems associated with producing a good speakers at a low cost!