I work in integrity at Meta. It's mostly about staying legally compliant with the countries we make $ in. You have to keep in mind we have like 4 billion users, and own 5 out of the top 10 most used apps (hello antitrust?), so it's a pretty big deal here.
Telegram, WhatsApp, and iMessage are all e2e encrypted but backups aren't. That's how your messages get read.
Messenger is actually the most secure platform out of the 4 because there's no unencrypted cloud backups. However, there's discussion on rolling back the encryption on Messenger because it's too good and causing UX pains.