The biggest reason for it is that in practice email isn't as open and decentralized as people like to think. For most of its history all advancements were controlled by Microsoft, AOL and Yahoo, and today it is controlled by Microsoft and Google.

Most of the OP list of things they don't like are about client UI presentation. Inherently unrelated to the protocol itself.

I don't have any of those issues because I use a client that does the interaction and presentation exactly how I like it.