cause user caches arent just used by the browsers, user caches are mostly used by the system, browser caches are used by browsers(BTW, cleaning out user caches also makes it so that when you start typing in a browser and it usually starts trying to fill in the url for you, if you clean user caches, it wont do it, itll be like reinstalling the os and having an unused browser.), and some unimportant(which are usually important in one way or another) are kept in user caches, very few setttings, also some directory access and such, so its like breaking in your computer again, kinda.