Wordpress requires some degree of setup, with the end users simply accessing the installed web application. With Gatsby, if you're at the point where you're ready to deploy, you've already done most of the technical work yourself, so the value add is far diminished vs managed Wordpress hosting.
this is true in some sense; the value of JAM-stack apps is that they are portable and deployment is vastly simplified and requires limited resources (web-accessible file storage). How much value-add is there to be captured?
Netlify already gives me everything I need for free at this scale. Once I move into their enterprise features the competition heats up.