Its really easy to make a custom rom but hard to do serious "real life" stuff; companies don't want to make it easy. To most regular users, if they cant download apps from the google play store, and they can't use venmo\cashapp, then the OS is dead in the water from day 1
Trying to get some scale, you're hypothesizing about giving 10 millions to HSBC to make business with your startup, when they're throwing away 500+ millions every year just to cover their money laundering.
https://www.investopedia.com/stock-analysis/2013/investing-n...
And we're discussing doing this for basically every major banks.