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Edit: Less snarkily, I think there just aren't a lot of bleeding-edge tech people in civil service, at least in the US. Would you want to give up your cushy dev job to work for a huge bureaucracy at maybe 40% of the industry pay?
So you're left with bureaucrats/administrative workers who grew up on MS Office and Adobe Acrobat, and who can't easily switch to new tools even if they wanted to without a horribly complicated purchasing process.
Some other countries and small localities experimented with replacing MS Office with OpenOffice, for example, or Windows with Linux but I think most of those failed. Similarly, most government geographical services use ESRI software even though open-source ones have been around for a while. Sometimes it's a matter of feature parity, but more likely than not, only big entities like that are equipped to deal with government contracting and support.
Government moves really, really slow, and prefers to work with other huge companies under really complex contracts... it's antithetical to most software/web work. Nobody is going to jump through all those hoops just to get us a slightly more standard PDF form. If you push them too hard they'll probably just go back to paper forms and fax, lol.
However, PDF also has a bunch of different versions and substandards, and different editors and viewers will support different features: forms, accessibility, encryption, Javascript, layers, embedded Adobe Illustrator data, etc. https://en.wikipedia.org/wiki/PDF#Additional_features
Acrobat probably supports the most features out-of-the-box, and only Illustrator can open Illustrator-in-PDFs, I think. For example, many government forms won't work right in third-party viewers. Like this worker's comp form: https://www.dol.gov/sites/dolgov/files/owcp/regs/compliance/... will have working fields in the Chrome viewer or macOS Preview, but the Print/Save/Reset form buttons at the bottom do nothing unless you open it in Acrobat. Or GeoPDFs, a third-party extension (https://terragotech.com/resources/sample-geopdfs/) that don't work right at all outside of Acrobat, even though they're used by the USGS (https://www.usgs.gov/faqs/what-geopdfr).
It's kinda like the Word doc situation of the 2000s... third-party utilities can usually open them and get the basic document and formatting, but not perfectly, and some features just won't work.