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.
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).
Stealing your furniture is theft. Taking a look at your furniture, and carving another set exactly like it is copyright infringement.