Yes, it's.... Yes is the short answer.
Payments are a funny thing. As you say, you look at it and, sure, it's a payment. How hard can it be? When you layer on the compliance considerations, the currency considerations, all of the different payment methods in different countries, the need for fraud prevention and the importance of self-serve access....
Traditionally, you'd have to go to the bank, plead your case, fill out paperwork and so on. Stripe pioneered this instantly enabling, self-serve model whereby a business can just fill out the form on a computer at home and instantly go live. This was a pioneering thing that Stripe developed. All of these together are very expensive to develop.
I agree that it sounds a bit funny when you first hear it. How could it be that hard?