1. There are lots of regulations that say only banks can do certain actions, like lend in all 50 states under the rules of a single state. Or open a FDIC insured checking account. Or have a unique account+routing number for each user to send ACH funds to (various reasons this could be preferred to everyone ACH to one single global account). These are valuable services without any debit card issued by the fintech.
2. It is basically impossible to become a bank. The government only approves a handful a year. Square (block) actually got approval recently but it is very difficult to do.
As a result of (1) + (2) is that if your company needs any banking products at all you need to partner with a bank because there is just no reasonable way to legally build that functionality yourself.