I have a few theories why
1. The people with the ideas lack the skills to build a decent site, but have just enough determination to slap a website together that serves a need, but is generally poor. Page weight, navigation, and browser compatibility never cross their minds. They are just happy to have a webpage online. Now, how do I setup a domain name?
2. The allure of the quick buck. Minimal effort to make money from from ads under the guise of evergreen content. This had it's day, but is mostly stomped out now. How many websites were slapped together with the least amount of work possible and left to rot in the cyber dump with individual pages of 500 word generic content in hopes of a decent Google ranking. Which leads me to....
3. Search Engines. As they grew in popularity, people realized that it was the least expensive way to get traffic. The game was on. No need to pay attention to users and repeat traffic, just figure out what it takes to rank well on MSN, Yahoo, and Google. Then watch the money pour in! Auto generated sites, scraping, link campaigns, and even ruthless content stealing became attractive to make a quick buck. When Larry and Sergey created link popularity, I wonder if they ever thought it would come to this.
eBay is a marvel to me. I think they got so many things right. Not sure if it was luck or skill. They made a platform that let the webmaster novices sell things online (took away the technical requirement to sell things online), then the incentive to create good content and descriptions enhanced a persons ability to sell. This aligned ebay's interest of quality with the sellers. Then the auction piece. Time based listings since so many people had decent products under a category it worked. Think if Google used time based listings. Wierd. Ever wonder what the percieved quality difference is between rankings on Google? Oh yeah, I'd always pick lucky over good. Ever here of Pierre Omidyar, I bet he is a little of both.