What guided Google's decision back in '98 was to deliver the most kickass search engine ever, the sooner the better (I guess). From that point to now it's a matter of perfectioning every single aspect of their creature, to ensure a great UX (therfore fidelization).

What makes a great UX? A clean UI, a fast service and possibly no hiccups (to make it very short). The UI has changed a lot from 98 to now (check out web.archive.org) and so did their HTML code. A single line HTML page is obviously a conscious decision and so is the inline CSS.

I don't know you, so I have no idea on your experience in this field, but when you get to deal with I'm not saying huge, but decent amount of users you simply have to deal with scalability under every single aspect. If you don't, chances are you end up delivering a crappy service OR you spend WAY more money (bandwidth, cpu power, etc) than you actually need to.

EDIT: google is obviously more than html and css, and if you really want to have an idea on how they are paranoid with performances you should check out their google app engine documentation.