It’s a widespread misconception that banks have marked most of their assets to market. The mark to market rules generally apply only to securities, not whole loans. Whole loans are carried at original face value, less any impairment (i.e., provisions for loan losses) judged appropriate by management, with input from auditors.
So the big banks have marked only a small fraction of their assets to market. In crushing all of the big bank stocks until the March 10 bottom, the market was discounting massive increases in loan loss provisions. The banks have already increased their provisions dramatically (in line with delinquencies and other indicators of impairment), but they still have a long way to go to offset the enormous potential losses on whole loans.