PayPal could at least offer the option for purchases funded entirely from existing funds in the buyer's account. That they don't suggests that protecting buyers is more valuable to them than protecting sellers.
Debit transactions have the same sort of protections.
The main difference between debit and credit is that if you're the victim of fraud with a debit card, money has been taken from your bank account and you have to fight to get it back. The fight is usually not very hard, but it takes some time. If it hits at the wrong moment you may have trouble buying food or paying rent on time, even though you'll get the money back before too long.
