The issue with that is that email is insecure because of the interoperability. When you have to trust multiple parties to handle the encryption it is much more likely that at least one will do it wrong or give in to government pressure and allow access to everything passing through their platforms.
For a platform to be secure you have to be able to trust everyone involved including the provider of the platform, the applications used, and the other users you communicate with.
For email this was all considered too much of a hassle and email isn’t encrypted in any other way than while in transit and that is only if all servers it’s going through has support for it. For higher level of security users must make an effort to set up encryption that works with the other parties taking part in the exchange of messages.
