I think most people who are not in this group would expect it to be private. My snail mail isn't encrypted, but I have an expectation that it is private.
Yes it is. USPS first class (ie regular not bulk) mail IS absolutely private and may not be opened by authorities without a warrant (and tampering with them by non authorities is a federal crime).
What security agencies have done is scan and collect all envelope info (metadata) which was ruled to be not private (makes sense) but with powerful enough cameras, lights, and enhancement software that most letters sent in standard envelopes can leak their contents.
I live in the US and have had snail mail opened without my permission and forwarded to me at the post office before. Once they put a sticker on it, once they didn't even bother. So, no, your snail mail isn't private at all. Government is free to open it and examine it any time they want. Legally the inspector is only supposed to open discount classes of mail, which is about half of them, to make sure you aren't lying to ship something cheaper, but in practice they are free to open anything and do.
Regardless of what your expectations are, it's not private. Just because you expect something to be private does not mean that it actually is.