First off, because it isn't. There are a number of things that personally shouldn't be styled. Or rather, there are a number of things that should have their st

First off, because it isn't. There are a number of things that personally shouldn't be styled. Or rather, there are a number of things that should have their styles overridable without breaking things. There are an astounding number of things I've seen abused, badly. I mean - look at the number of websites that aren't usable without a web font because they embedded all of their icons in it. Bonus points if they load the web font via JS. Bonus points if it's third-party JS. Secondly, you cannot style every other UI component except this one. The browser chrome. The mouse pointer. The context menu (to an extent, depending on browser). The thing with scrollbars is that they are at the boundary between the website and the browser. There are pros and cons - allowing "good" websites to apply "good" styles, but not for inane UIs. Personally, I think that the better approach would be for the browser to color the scrollbar dependent on the background color of the page.