Apparently first mover advantage (with about a decade head start?) still counts for a lot. https://www.chargeflow.io/blog/paypal-statistics-facts#paypa...

That estimates PayPal still has 40%+ online payment market share.

I'm assuming a lot of that is long-tail 'I put a PayPal link on my store in 2005' stuff, but I imagine those add up.

And they're also probably a sticky customer base, in the sense that anyone still using PayPal as primary today doesn't like changing things, which makes acquiring those customers worth it.