> swedes, turnips, cabbage, beetroots, leeks, rhubarb, types of squash, etc.
Perhaps this is a U.S. issue. Every single one of these is available at my local Lidl supermarket (~a marginally upscale Aldi for those outside Europe) for much of the year, not to mention other more starkly seasonal things that filter in like peaches, salsify, runner beans, cherimoya (a tropical fruit), and so on. Across Europe I've had no trouble getting all the offal I'd want: head cheese/zult, haggis, black pudding, balkenbrij, hearts, various horrors suspended in aspic-- in supermarkets. Not specialist shops, not the butcher. I'm in rural Ireland now, but the selection was better yet when I lived in Switzerland and NL and also shopped at Lidl.
Those are all available in any US supermarket well. The quantities on the shelf suggest they don't sell well, just enough to be worth having.
I should have mentioned that I am in the UK.
Many of them tend to be available in supermarkets, though it varies by location, but, really it's only older people who buy them.
That said, coming from the continent myself, the UK is below average. For an island the general lack of a variety of seafood is especially stricking.