Because they are not living in the real world. I assume these people are just getting paid to write a proposal and then others are paid to write code for the proposal. They do that, then they go home. Then other people see it and get excited and play with it and think it's cool, then go back to using email. None of the parts involved at any point really think this will be actually used by anyone in reality.

Also there is no way to do reliable rotation or key hierarchies without a centralized authority (or a blockchain) and a master key.