[kevincox]

Because it destorys native browser features. For example find-in-page, hove to see where a link goes. Right click a link to bookmark, open in private window or copy the URL. If the page has anchors I can get a link to them. If I save or print the page it is an image rather than text (with all of the above features).

Some of these can be reimplemented easily, some of them with effort, but some just aren't possible. Plus the exact behaviours depend on what browser you are using, so you can't reimplement them because you don't know how the user's browser behaves.

For games or image editors this is probably fine. But for most apps this is a huge depredation in usability.