I think it’s that PayPal was one of the firsts to provide a method for collecting credit card transactions electronically.
Before PayPal, you’d often have to visit a website, then call the phone number for the seller to collect payment.
eBay needed paypal because their sellers were often not businesses, just people yardsaling stuff online.
Coincidentally, I interned at a PayPal competitor in 1998 that went under during the bust. We had an electronic interface through MS access, but it was a still a human entering in the CC number into one of those dial pads on our side and then confirming the transaction. I’m sure with all of the concerns around security nowadays that you can understand why that was a terrible long term business model.