Why web designers should lean to the left. Not politically but how the design web sites

Nielson Norman's latest newsletter is about the way that we have a natural inclination to pay more attention to the lefthand side of a web page.

Web users spend 80% of their time viewing the left half of the page and 20% viewing the right half. Adhering to design conventions will help maximize users’ efficiency and company profits.

This is one of the most simple, easy to understand, pieces of UX knowledge, that has immediate application to improving the way web sites are designed. I wonder why so many designers ignore it or try and prove it wrong?  Dick Stroud