WhatsApp is huge in many countries. You and I may have the tech skill and privacy concerns to move to something like signal but 99% of users either don't care or aren't aware of the difference.
Adopting signal was exactly as technologically challenging as adopting WhatsApp for me. The desktop client is harder since it's not a web service, but no one I know that isn't 'tech savvy' uses the web client for Whatsapp either.