It just feels like the baseline expected behavior of a communications system to me that makes no explicit claims otherwise.

Legal signatures are heavily ritualized (blue/black ink only, initial here and sign there etc.) in most societies for good reason – it makes the signer stop for a moment and reconsider what they are doing, if the document they are signing is truly aligned with their intentions and so on.

As another analogy/food for thought: We have the technical means to record every conversation we ever have, digital or analog, public or private. Should we? If not, why not?

> It just feels like the baseline expected behavior of a communications system to me that makes no explicit claims otherwise.

Why do you feel like email "makes no explicit claims" about the authenticity of emails? Laypeople are not even aware of the possibility of spoofing the sender field in emails. Technical people can check the "explicit claims" of a protocol like e-mail, SPF, DKIM, etc. to understand what it claims to do. In other words, email makes both implicit claims, and explicit claims about the verifiability of the sender field.