They gain profits that make them rich because of interest or spread based income. They also get profits from sources outside of banking services.
Income from interest is the main priority of the bank. People or customers who want to take loans are very concerned about them because they, as the bank, know they will get a lot of returns with a set interest percentage in a matter of time.

That is why many people are unable and willing to cooperate with official financial institutions under the legal auspices of the government because they do not want their lives to be pressured by the banking system.