There already are: http://www.theguardian.com/technology/2013/aug/26/bitcoin-lo...

> Now that is a clean, friendly design.

I was pleasantly surprised to find that the header wasn't fixed. I can't wait until that fad dies out.

What is wrong with a fixed header?

Isn't it more convenient than scrolling back to the top of the page to access important links?

The only possible issue I can think of is if screen space is limited.

Fixed headers consume vertical screen space -- permanently. Instead of allowing you to scroll down to have maximum vertical space for the content.