We don't seem to have a Public Key Infrastructure that makes digital
(cryptographic) signatures useful for non-computer work.
We do, but it might not be a "we" that intersects with our "we". The building industry in Ontario has been using digital contracts (real ones, not that cryptocurrency sham nonsense) for some years. The PKI has been managed by one of the building trades associations, who (unsurprisingly) see it as a revenue stream and charge accordingly. Consequently, it is beyond the reach of mere mortals.
Digital signatures - even self-signed signatures created in Adobe tools and some others - are as valid in Ontario as wet signatures.
One of the weirdest places I found a full PKI setup in frequent (if unwitting) use is in ARRL's Logbook of the World (LoTW). Thousands of ham radio operators submit their contact logs to LoTW, and they're all cryptographically signed. Every user gets their own P12 certificate file. Unfortunately, ARRL's certificate isn't distributed with any browser or PDF reader, so it's not generally useful as a signing tool. Some years ago, I did try to get amateur radio types to be interested in using the LoTW certificate to authenticate other files, but it never caught on. The PortableSigner Java application was how I added signatures to PDFs.
Stewart