you can encrypt the content but not the metadata, not even the subject unless you use a customized client that encodes it (like deltachat which doesn't use a subject at all), but then you still have your email address exposed.

for all intents and purposes email is not e2ee.

I can bring my own encryption to tiktok as well. Has roughly the same usability and usage.

you can bring your own encryption to ANY messaging platform, doesn't mean it will be easy to use. e2ee just really makes it handy so that users don't need to preshare any keys.