While it might be disappointing, I firmly believe this is a technical and business decision, not a conspiracy. If you look at the features Messenger offers, and the direction the product has been moving, it relies heavily on server-side technology.

Facebook have added contextual ride-share ordering, person-to-person payments, bots, etc. Unlike WhatsApp or Signal, Messenger also still works over the Web without an app or a smartphone-based login: how do you implement credible E2E over the Web, without using the phone as a crutch? How do you allow multi-device support, with a message history, and do E2E on all conversations?

I think they've (rightly, IMO) assessed that most people value those features and convenience over 100% E2E conversations, but want to offer the option to those who don't. Besides, if Facebook were really completely in bed with the surveillance state, why would they have just rolled out full E2E on WhatsApp?