Nobody has looked at his loan applications and his tax filings at the same time. You can estimate the values of your assets at the high end for loan applications, and you can estimate the values of your assets at the low end for tax filings. Do both in the same year for the same assets, and you have committed bank fraud and tax fraud. Trumps standard MO for years has been to keep everything compartmentalized, so one authority does not know what he told the other. That could end soon. His fixer testified under oath that he did this. Cohen may have been one of the few people that were aware of both.
And yes: People do go to jail for these crimes, particularly the bank fraud. There are people at Deutsche Bank who would testify they thought he was committing bank fraud.