WhatsApp was its own company, took advantage of an open market in EU where SMS (and "international" phone calls?) were extra rate charges on mobile phones. Once every one got accustomed to using whatsapp Meta took it over and now we're stuck with it.
It's almost impossible not to sadly, at least if you want to reach everybody in ypur contacts... It was the first popular messaging app here and inertia prevents people from moving to better alternatives now.
The reason is the network effect. I want to use signal or rather even an EU based messaging service, but everybody, including businesses, are on WhatsApp in my country.
In an ideal world, maybe. Hopefully we will see more of that thanks to RCS, since major players like WhatsApp, Message (Facebook) and iMessage will have to open up and be interoperable with other messaging systems.
So far, though, I'm stuck with WhatsApp (and iMessage for just one person!) and Telegram, nobody uses Signal here