Because part of PayPal's service to their customers is that you can dispute a transaction and they will work to resolve it. It's one of the features that protects (and therefore attracts) consumers. Clearly, it causes negatives for sellers. It's a trade-off.
Consumers want chargebacks. They let you get your money back when you get scammed or your payment information is stolen. Does that never happen in Europe? Nobody ever takes your money then fails to deliver the product/service? You never have your information stolen and used to pay for something you didn't authorize?
Please don't play dumb; it's not the appropriate level of discourse here. Is your police department going to open an international investigation into how many USB cables a Canadian company shipped or didn't ship to you? You know that realistically, if you wire money to an account of someone untrustworthy, you have no recourse at all. There's no guarantee there's even an account on the other end to recover money from.
With credit cards, millions of people pay millions of merchants they have never done business with before while completely protected against fraud. The chargeback dispute process is completely international; the burden of proof is on the merchant no matter where in the world they are. And the protection is guaranteed for months, even if the merchant has disappeared.