I'll echo everyone else here that has said a lot of websites and companies do have this. But the reason not every one of them does is that it is extremely time

I'll echo everyone else here that has said a lot of websites and companies do have this. But the reason not every one of them does is that it is extremely time consuming, takes a lot of thought, and you have to spend time enforcing the guidelines once they're created. If you're not committed to following them, it's a waste of time and money to define them. Most start ups are worried about things other than logos and colors matching across multiple mediums (print, web, broadcast, etc.) because most web sites and web start ups only have one medium: the browser.