Most parents I know dont even allow their children WhatsApp (as you said there are safer alternatives like Signal without weird social network functions you can barely control) so why would parents use it?
For me this just sounds nonsensical. I am sure it makes sense somewhere in the world
Where I live, WhatsApp is the standard. 100% of people have it installed, minus some hermits who have never heard of the Internet.
My parents know how to use WhatsApp and have it installed. My sister and her family, too. My friends, their friends, the scammer trying to explain to me how to access the crypto I apparently lost track of on their platform, my former coworkers, my current coworkers, the people I interact with for my freelance work, the government's road safety alerts, my ex-girlfriend who still has a bunch of my clothes, my neighborhthey areood group informing me when I need to put out the trash-they're all on WhatsApp.
It's the network effect. Even to get my family group on Signal, I'd have to get twenty people to install Signal, probably visit some of them to help them with the setup, do it again every time they switch phones, and so on. And they'd probably still default to WhatsApp because that's what they know.
I mean yeah, I'm sure it depends and varies a lot. I would feel more comfortable if they used something more privacy-oriented, and I do the same. The rest, of course, is up to me.
I do not think I would be able to get my kid off the grid completely so might as well use a better alternative, and in many cases you simply just can't because you have to submit your homework through Facebook, and so forth. Thankfully it has settled down a bit post-COVID but there may still be cases of this crap. I loathe it.
In any case, a privacy-oriented alternative is better than the cancerous Meta. If you want no IMs at all, well, that may be tough. Kids find a way, trust me. I know I did. :D
For me this just sounds nonsensical. I am sure it makes sense somewhere in the world