That's just a USA thing, rest of the world use 3rd party platforms to chat.
For instance, SMS seems to be preferred for online selling platforms in which only a phone number is exchanged. It's strange to add a stranger to your contacts for a one-time transaction, so I've often been contacted by SMS or email.
It might also help that it's increasingly common to pay over the TWINT mobile payment system created by Swiss banks for which mobile numbers remain the primary identification method for private users, not to mention SMS messages used for transaction confirmations or login codes. Given the default option of a phone number for many of these private sales, an SMS is probably more straightforward than adding a new contact to use another app.