Because it's like email: It's guaranteed that 99.9% of people meet can send/receive it, for free. It's far easier to establish communication instead of playing

Because it's like email: It's guaranteed that 99.9% of people meet can send/receive it, for free. It's far easier to establish communication instead of playing the platform-negotiation game then trying to remember where to message who. Out of the examples you listed, personally, I don't use FB properties (so no WhatsApp or Messenger), nobody here uses WeChat/Line, Telegram is gross (hard pass on closed source faux-secure messaging), and I've never even heard of Viber. But I use Signal, and it's integrated with SMS (in the Signal app, it'll use Signal to talk with other Signal users automatically, and fallback to SMS otherwise). And yeah, SMS is basically free (read: unlimited usage included in pretty much every phone plan on every carrier) in North America.