Um, because for developers, integrating with PayPal and dealing with the payment issues it (potentially) entails sucks compared to integrating and dealing with a modern, credit card-only payment system like Stripe.
Why exactly do you care so much about giving your credit card details? You have, typically, $0 liability for unauthorized charges on your card.
The only thing I think about is the (minor) hassle of dealing with getting unauthorized charges reversed in the highly unlikely scenario that your credit card is stolen from the site at which you just paid. I compare that with the potentially major hassle getting PayPal to put money back in my bank account that was transfered out by a hacker. I'll take the credit card companies over PayPal any day.