It's because Stripe negotiate steep discounts. The APIs are merely the trojan horse to get inside your org.
Speaking from experience at the table at some of these deals, Adyen or other competitors would often approach my employer (large e-commerce company) offering a better deal, but executives would merely take the deal to Stripe and say "match it or we're leaving", and they would.
Given the cost of leaving is only somewhere between $50-200k depending on the complexity of your checkout systems, the client base isn't particularly sticky.
I'm bearish on Stripe for this reason. If they're just racing to the bottom with Adyen and others then where are they going to make money? Their other products are pure garbage (Stripe Radar is worse than useless).