Because your memory is romanticizing the past to a large degree. The early days of the Internet were basically text only, and you had to memorize a number of commands to get around. Then came the web, and it was just static text and images. Then someone hit on the idea of the animated GIF, and as HDD space increased, so did the resolution of other images. Then along comes Google with Gmail, making use of a little known, up to then, function of JavaScript to reduce the number of times you need to refresh a page. Before then, if you had something like gmail, you'd load the main page, then if you wanted to read an email you'd click on it, and it'd load a whole new page. Along the way came morons who created giant flash apps instead of actual web pages, Java craplets (which seem to have largely died out client side), and all sorts of other resource intensive stuff. Resolution of flash videos has gone up, the amount of "interactive" content on web pages has gone up, there's a lot more secret tracking of your every move on the web these days via embedded scripts (these forums have scripts from no less than 10 different domains being loaded and domains like visualrevenue.com don't exactly instill me with a lot of confidence that they're seeking to somehow improve my experience on the site).

Plus, things have been advancing on the client side as well. No one wants to buy the latest version of Windows if the only major feature is that Microsoft squashed a lot of bugs and fine tuned performance. If they tried pushing something like that, it would make the marketing disaster of Vista look like nothing. We all want hundreds of new features, even if we will probably never use 99.99999999999% of them a single time. Those features require resources, whether we are using them or not, so the hardware has to advance to keep up. Plus Intel and AMD are interested in getting you to continually buy new chips, otherwise they won't keep making money and will go out of business. So depending on how you want to look at it, it's either a vicious or virtuous cycle.