One reason that comes to mind is that a lot of people browse on mobile devices. These days a site really ought to be responsive to mobile viewports and a lot of “basic bitch” government sites are not.

Plain old HTML is responsive. It stops being responsive when you start trying to make it look good.

Plain old HTML stops being responsive as soon as you try to do anything more than text. A plain HTML table would be horrible on mobile. If you want to reflow anything that isn't text you will need flexbox. There is literally no reason to not use CSS to make a site that works well on mobile and desktop.

I’m not convinced that’s true. Case in point: tabular data. I don’t think presenting something in a table is an attempt to make it look good, I think it’s the most logical and recognisable way of presenting certain kinds of information. But tables don’t scale well on a mobile viewport without extra work.

Reasonable tables work fine on a mobile viewport. It’s when you get into crazy ones that it breaks down.

Part of function is layout. It isn't any more difficult to grab Bootstrap or some other out of the box framework that'll give a good consistent look than it is to go "no style".

Or jquery, the original IE glue library.

There can be a medium. US Gov sites generally look awful. The UK Gov Digital team have done a way better just of standardizing things in ways that look nice.