the fact that many of the things you need WP plugins for are standard in Ghost does explain some of the differences between the theme markets for the two products I think.
I suppose one thing that would be equivalent to the WP theme market is where companies offer suites of themes on a single licence so that people can experiment with multiple views, from one company. I have used GeneratePress Premium - excellent, btw! - in the past and that’s what they offered (in addition to the licensed plugin that allowed for some customisations not available in the free version).
Of course, there would be nothing to stop Ghost folk providing a basic open source theme and then licensing the paid versions on different terms, or open sourcing the whole lot - while charging for support - if that’s what they wanted to do.