Because corporate sites typically have to obey a wide range of laws (accessibility, compliance, etc), support a wide range of browsers (ie6), support multiple languages, and meet corporate design guidelines (which cover a wide range of material including printed brochures, tv adverts, etc).

They also tend to be redesigned every 3-7 years because they're huge multimillion dollar projects due to their size and complexity, hence they can't just follow current fads. They need to follow a design which will still be approriate in a decades time.