It struck me recently that most cars are increasingly looking the same. It used to be that Peugeots had a very typical nose, Volkswagens had their own boring but efficient look, Volvos were kinda blocky. These days I can barely tell the make of a car by looking at the lines, and that's because they all need to perform the same function: transport a similar number of people at similar speed with as low fuel consumption as possible. So they all have basically the same streamlined lines.
Most websites want their navigation to be found, so they put it in the same place as every other website. They want their story easily readable in the center.
There's still plenty of variation, but sites with similar goals are going to end up with a lot of similar elements.
