As you can see, there is no way a bank could charge the kind of interest necessary to achieve even a fraction of that potential return. Second, Banks are highly regulated. As someone who has been through bank exams, let me assure you that a bank would never be allowed to continue to make that kind of loan. If the bank recognizes the risk of these start-up ventures, the examiners will look at those loans in every exam, and the bank will be criticized for having made the loans. The banks could fail to categorize the loans as risky, but any loan reviewed by an examiner would not only be criticized for risk, but for failure to rate the risk adequately.
Banks live and die by their exams.
If a bank takes unwarranted risk, the examiners can close it. Basically, there is no good reason for a bank to take the risk involved in loans to most start-up companies. They have all the risk of a venture capital firm, but a potential net return of roughly what you get on your savings account. Banks can, and do, play a roll in small business. Banks work with government guaranteed programs to provide loans for these kinds of ventures.