Why are encrypted native SMS clients not a bigger thing?

What’s preventing people from using a third party sms client (or even just a keyboard) that auto-encrypts the contents of the sms as it goes back and forth?

I get that a separate channel is more profitable, more controllable, but is that the only reason this concept hasn’t already taken off?

I see a few tools on the app stores that do this, but clearly no big players yet (afaik)