Because other companies that offer similar services offer this feature, and thats why people use it and trust it. Paypal is very buyer focused as opposed to seller focused. Eg. if you buy something on ebay and you don't get the product or its defective or its fraud, paypal reverses the tx and puts the onus on the seller to prove you wrong.
This doesn't matter so much when sending money to friends, but an expected use case people imagine for facebook based p2p tx is fb marketplace, where you can buy things from random people online. A lot of fraud and sketchy behavior already happens there (eg. buying stolen products). Not being able to have easy reversion of such tx is bad for buyers (and good for fraudulent sellers).
Not sure if you're in the USA, but for US users, banks are pretty hands off this sort of behavior and will only reverse TX in serious fraud issues, but require the loser to pass a high bar to prove its fraud. Normal bank to bank tx is a "pull" model, so middle men services typically offer protection for the buyer to stop a "pull" in the case of fraud. Thats why the US has created many p2p services that are "push" based (eg. venmo, cash app), and why credit cards companies offer protections like charge reversal and take on that risk (you agree that you'll return money in a charge-back as a condition of VISA/etc letting you use the network).