Because a bank robbery is a robbery, not mere theft. In a robbery, you make a credible threat to use force against another human being. That's hopefully still worse than stealing $x.
You would have other charges added in addition to the actual taking of the money which related to how threatening you were at the time of the robbery. I'm saying the actually robbery charges being on par with distributing copyright content and ultimately taking money out of the pockets of the content creators are, at the end of the day, both the same if the same amount of money is taken.
A bank robbery still has about as much in common with copyright violation as with pirating actual ships. I am not threatening anyone when committing copyright violations
A much more convincing argument could be made that theft and copyright violations are the same. Buy theft has quite low punishments (at least in California).
..if that argument were not destroyed by the Supreme Court many years ago. Infringement is not theft, no matter how much big businesses want to equivocate the two in people's minds.