Because we're talking about the backend-deployment+ops-jargon terms "website" and "webapp", not their general usage. Words can have precise jargon meanings which are different in different disciplines. This is where ops people tend to draw the line: a website is something you can deploy to e.g. an S3 bucket and it'll be fully functional, with no other dependencies that you have to maintain for it. A webapp is something that does have such dependencies that you need to set up and maintain—e.g. a database layer.

But even ignoring that, I also define the terms this way because of the prefix "web." A webapp isn't "an app on the web", but rather "an app powered by the web." An entirely-offline JavaScript SPA that is just served over the web, isn't a web-app. It's just a program that runs in a browser, just like a Flash or ActiveX or Java applet is a program that runs in a browser. (Is a Flash game a "web game"? It's usually considered a browser game, but that's not the same thing.)

We already have a term for the thing that {Flash, ActiveX, Java} applets are: apps. Offline JavaScript SPAs are just apps too. We don't need to add the prefix "web"; it's meaningless here. In any of those cases, if you took the exact same program, and slammed it into an Electron wrapper instead of into a domain-fronted S3 bucket, it would clearly not be a "web app" in any sense. Your SPA would just be "a JavaScript app that uses a browser DOM as its graphics toolkit." Well, that's just as true before you put it in the Electron wrapper.

So "web app", then, has a specific meaning, above and beyond "app." You need something extra. That something extra is a backend, which your browser—driven by the app's logic—interacts with over the web. That's what makes an app "a web app." (This definition intentionally encompasses both server-rendered dynamic HTML, and client-rendered JavaScript SPA apps. You don't need a frontend app; you just need a web backend that something is interacting with. That something can be the browser directly, by clicking links and submitting forms; or it can be a JavaScript frontend, using AJAX.)

A "web site", then, is a "web app" without the "app" part. If it's clear in the above definition what an "app" is, and what a "web app" is, then you can subtract one from the other to derive a definition of a "web not-app." That's a website: something powered by a web backend, which does not do any app things. If we decide that "app things" are basically "storing state", then a "site" is an "app" with no persistent state.

And since the definition of "web" here is about a backend, then the difference between a "web app" and a "web site" (a web not-app) is probably defined by the properties of the backend. So the difference about the ability of the web backend to store state. So a "web site" is a "web app" where the backend does no app things—i.e., stores no state.