I have a friend group that insist on all events being planned through facebook.
I’ve missed out on events in the past due to not taking part.
It’s no longer a hill I wish to die on.

I agree and use Signal myself.
But people like the extra features of WhatsApp like desktop/web clients with seamless history sync and all the other little things that WhatsApp provides.
The average Joe doesn’t even think about security or privacy, they just know that the results of using WhatsApp are superior than using SMS.
iMessage is a non starter everywhere out of the US, it just doesn’t have the market penetration.
As an Australian, no one I know (many of whom own iPhones) talk about the blue-green bubble stuff.
They recognise where the fault lies and simply don’t use the app.

In certain places like India, WhatsApp is the default means of communication for everyone.
You can use it without phone data if you are on wifi, it supports better quality than sms for sending images, you can video chat with it, it’s cross platform, etc etc.

What’s more amazing to me is that it’s not more popular in western countries.