Because there is no alternative and they fucking know it.
Oh if you're in the US for the US you might be able (and want) to gamble on an other younger service, but in the wider world? It's a fucking wasteland, it's either Paypal or Big Bank Merchant Account, and those are no fun at all either.
My card expired (and was renewed) recently, I decided to stop using paypal (so I did not update the account to the new card), by the third christmas gift I tried to get I had to re-activate it, because either the gift in question wasn't available from anybody not using Paypal (etsy merchants, for instance) or because the CC gateway alternatives to paypal were unusable to plain broken.