Although the web platform largely isolates different origins from each other, there are a number of "leaks" whereby one origin can extract information from another origin. For example, browsers let one origin embed images from another origin, leaking information such as the height and width of the images across origins. These leaks are often at the core of security vulnerabilities in the platform.
These same leak exists, of course, between local origins (e.g., those with file URIs) and non-local origins (e.g., those with http or https URIs). What kind of information could a web site extract from your local system using this leak?
On my laptop, I have Skype installed, which means that, on my laptop, the URI below resolves to a PNG image with a particular height and width:
file:///Applications/Skype.app/Contents/Resources/SmallBlackDot.pngIf I visit a web site, if the browser doesn't address this leak, the web site could determine whether I have Skype installed by attempting to load that URI as an image. On my laptop, the image element would have a certain well-known height and width, but on a laptop without Skype installed, the browser would fire the error event.
Returning to Cedric's question, why do browser vendors restrict access to local URIs but not to non-local URIs if both have the same information leak? I would prefer to close this leak in both cases, but many web sites embed cross-origin images, e.g. from content delivery networks. If we were adding the <img> tag today, we would probably require servers opt in to cross-origin embedding using the Cross-Origin Resource Sharing protocol.
Fortunately, very few web sites include images (or other resources) from local URIs (especially after we removed the full path from <input type="file">, but that's a story for another time). That means browsers can block all loads of local resources by non-local origins without making users sad, preventing web sites from snooping on your local file system.