WhatsApp is a messenger. Facebook, Instagram and Threads are social media. Also, Threads is actually by Instagram moreso than it is by Meta (although the distinction isn’t too dramatic, it’s just under the Instagram branding)

The fact that WhatsApp is so crucial may very well be the reason. In India and places in Europe, WhatsApp is literally a direct replacement of texting, which means it contains communications from practically everyone and everything - your workplace, your local government, your grocery store, your gym, your friends and family, public services, etc. And since your chats themselves are E2E encrypted, the background usage data Meta can extract from users will be too dirty and unmonetizable.

Their interactions on Facebook and Instagram - now that’s rich data. They get to know exactly where and with which people your preferences and interests lie.