[vorpalhex]

Well, you're going to need apache or nginx, then you're going to need a php cgi or fpm. You still need somewhere to store your data so now you get a database layer that has to scale too. Don't forget your op cache. Oh and you probably need some kind of shared filesystem too, so there's that.

If any of those pieces break, get overloaded or have a security bug, your website is down. When your website isn't getting traffic, you still get to pay for those things, maintain them and perform regular updates.

With a static site you have files that get served by whatever. As long as you are serving files you are up. You should update your nginx or whatever on occasion. Scalability is stupidly horizontal.