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I use WhatsApp to talk to friends in other nations who only use WhatsApp. I don't consider it to be a secure communications channel.

Care to elaborate? All I can find is that Meta bought a VPN company under that name.

Was it forced into Whatsapp somehow?

Billions of people using it creates a strong network effect...

Sure, network effects are hard to overcome; my point is that Zuckerberg bought it as a competition killing move so they're not likely to put any quality control into it

I guess you are entitled to your own opinion (which I disagree with, though I recommend using Signal because the clients are open source).

IMO the answer to your original question is: lock-in effect. People are generally too lazy to move to an alternative, and on top of that, if they decide moving to one, they don't necessarily know which one to chose. And many privacy-advocates don't help by being (again IMO) too extreme: "Do NOT use Signal, you MUST use <choose your complicated ultra-secure alternative here>".