They (designers) do it to keep you from sniffing their handycraft code with the inspector. (That’s a joke.)
The browser is no webpage-viewer-app any longer. Especially not when the “page” is actually a full-fledged webapp. As soon as an app begins to be an app viewer, it’s really become an OS. (At least the Web is the OS, and browsers implement the frontend layer.)
Imagine the right-click context menu on Windows or OSX would be the same, all times, in all apps. Not much “context”, no?
One could claim those browser right-click context menus hadn’t to be there at all: at least, they’ve been “polyfills” with generic defaults, awaiting the (right, contextual) functionality being implemented by the webapps themselves. Granted, they were useful, back when web pages were just static hypertext documents and browsers were mere html viewers.
Browsers will continue to play that schizofrenic role for some time, both as a document viewer, and as a UI renderer. But eventually we will need two separate “web surfing” apps.
This is also what’s going on, I guess, when you here people complaining about pervasive javascript, progressive enhancement, and their demand that all web “pages” must support their no-js browser plugins indifferently.
The Web is more than pages: we got webapps now.