There's a lot they can do, but then it makes it impossible to search your inbox for old emails by subject, from, to etc fields.

You can reduce what an adversary sees by not including anything useful in the subject line and the only thing you can't protect is who you're speaking to and when.

The third party doctrine[1] allows the government to access your call records (and other metadata) without a warrant, but I don't think anyone's fine with that.

[1] https://en.wikipedia.org/wiki/Smith_v._Maryland

Security / convenience tradeoff.

Not being able to access the email body already makes full text search impossible, among other things. It is one tradeoff for using Protonmail instead of, say, Gmail.

Encrypting header data would make most operations impossible unless you download your entire inbox first.

STARTLS usage is over 90% these days. So the email server(s) get your metadata but passive network observers do not.

So the providers can work to get that to 100% and mandatory to foil active attacks.