Guess you'll be even more surprised to learn that Venmo absolutely does chargebacks! Go read their user agreement:
"It is not possible to cancel a payment without the consent of the payee".
Aka, if i give you money and you say you don't want it, you can cancel the payment.
But on Amazon or Ebay, you pay me for an Xbox, I ship it, and 3 days later "IT'S BROKEN" and I lose all my profit + shipping fees