Robbery has an under 25% clearance rate, but bank robbery has an over 60% clearance rate—the median bank robber gets prison (yes, 80% of stolen funds aren’t recovered, but how much of a prison term js getting away with $7,500 worth?)
Robbery is often committed by a single person. Bank robbery generally requires multiple, and often results in multiple charges per crime (which can skew clearance rates)
My brother is a detective. One of the early, surprising discoveries in career is that most criminals are dumb, fully driven by their Id. Basically, people driven by getting drugs or getting laid.
Because as should be obvious to the most casual observer, assuming rationality combined with financially-return-focused utility functions produces a model of human behavior that bears basically no relationship to reality. (Which is more an answer to why the prior underlying your question is silly than it is an answer to the question, but...)
bank robbery is automatically upgraded to the FBI and has (IIRC) a minimum sentence of 7 years, worse if you use a real weapon. I'm sure there are other ways to commit a petty offense or 2 that would net the same amount with less risk.
Maybe I should switch careers…