I have over ten years experience and the distinction isn't always clear to me.

Example: This logistics SPA I was building I realized could just be single pages with some data for most of the stuff (tracking, inventory, etc...) but for admins they wanted a whole dashboard. This was a conditional on some value of the stored session user. So it ended up being kinda a website for parts of it and an SPA admin panel if the user conditionally matched some privileges. Probably should have been separate stacks but they used the same data so early on they made it the same Next app.

I don't think the whole website vs app thing is always as simple as static blog pages vs full fledged JS-heavy app. There is a spectrum and overlap even within a single "application" because of various requirements.

Your last sentence is the most accurate. I don't think its primarily ignorance, its just trying to meet all the requirements while retaining some level of organization in the codebase.