[yucky]

Tens of millions of businesses, and when you throw in their Venmo service you can probably double that number, although Venmo isn't typically used in B2C.

And although it's not hard to get a merchant account per se, it is 100x harder than getting a Paypal account which takes 5 minutes and doesn't require any underwriting or paperwork. So people and businesses still use it (and Stripe, Square etc) rather than going through the hassle of getting a merchant account.