If it's as difficult as they say it is, how did Stripe do it?

They worked at it for a long time before launch, started with a very narrow product and only had customers they actually knew (reduces fraud a lot), worked like crazy, operated in a market where the competition was abysmal.

There's an avalanche of money waiting for more competition to Nvidia too....

The problem is, many folks have absolutely no comprehension of how difficult it is to build a payment processor that can serve lots of customers they don't personally know. The fraud is just insane. The regulation is insane. The card network rules are insane.