Inscryption?

If you haven't looked recently, you might take another look.

I felt the same way when Slay the Spire came out in 2019 -- not a lot of similar games at the time, and I couldn't figure out why more developers hadn't made similar games, as it seemed like a very good match for indie studios. But there have been a whole lot of games that came out since then.

Searching Steam for games tagged as single-player and deckbuilder, and sorting by user review

I get over 600 hits, almost all of which came out in the past three years. I'd say that single-player deckbuilders -- and note that I'm assuming that you're talking about deckbuilder games, not, say, solitaire implementations or similar, as I think that there are pretty good entrants there -- are actually doing pretty well.

Slay the spire? It's a deck builder rouge like. Excellent game. Dunno if it's what your looking for but I highly recommend it.