Couple reasons:

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.

To communicate with anyone outside of the US, where it is extremely popular and is the main interaction with many businesses as well.

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.