Well, simply put... because your web browser isn't very good.

There are better browsers today than there were a few short years ago. They are free, and they will give you a superior internet experience to the browser you are currently using. Please upgrade your browser so that you can see the pages that are designed according to the current web standards. Trust me, you'll be happy you did.

Here's a link to the Web Standards Project's browser upgrade initiative, where you'll find further insight into why you should upgrade, and following this paragraph are links to a variety of good free browsers.

Firefox - Safe, Secure, Free

Opera Software - Free and USD $39 versions available

Netscape Navigator - Free

Cheers, and happy browsing!