no one cares that Stripe, Twilio, or Plaid aren't fully open source

You keep drawing parallels to fintech

Tbh, it's pretty much the same reason that made Plaid popular with the fintech ecosystem.

But it's not the same thing. It's impossible to legally implement this yourself. You can't just make a library (I, naively, did try). All that being said, I understand the product now. It's not for me, but that's ok. Thanks for being so responsive