I’m not sure of the reasons, I suspect it descends from older customs of writing “dearest” and the like among the upper classes in the 1800s… Those letters are always very flowery. Anyway, pure conjecture.

What I wanted to say is that I have seen a slip from “dear” to “hi” as the common email greeting in semi-formal spaces. I only ever see “dear” used for the very first email to a new contact, and even then relatively rarely.