Why aren't politicians and their staff using end-to-end encrypted VOIP?
I.e., it's because none of this privacy technology is built-in as the default into our communications infrastructure. Therefore, for the average person (and even for privacy-conscious techies), it is inconvenient and difficult, and you have to convince the party you're talking with to also install and configure something that is inconvenient and difficult for them.
I'm convinced that the question of why end-to-end crypto isn't the default is a political one; i.e., there is pressure not to do it.
Email is a special case because of the various semantics like searching and federated clients.
Some of the comments are worth reading if not for the sheer size of the discussion. It could take an afternoon or two to fully digest that thread.
The comments are in response to this: https://blog.filippo.io/giving-up-on-long-term-pgp/