Because (potentially) stealing $6 million is still a very large crime in the absolute sense, even if it is smaller than other related crimes. Because it would be bad to alert white-collar criminals to the fact that they can escape prosecution by not being the largest participant in the criminal enterprise. Because $6 million is roughly 1000x larger (not a typo) than the typical bank robbery, which carries a 20-year sentence, and we don't advocate for non-prosecution of $500 bank robberies. Because it's only fair in a justice system that routinely imprisons people for stealing much smaller amounts.