I think part of it has to do with the fact that web development is a very large and diverse field. It's very possible that the types of projects you work on do not require a different approach. For example a web application with a large about of UI elements. Or very specific client requirements. Or a continuous cycle of iterations over time. Or a common structure for a team.

Indeed the old web dev write once, pass it to the client and never see it again has quite a bit of difference in requirements to the large scale robust web application that needs to be able to survive a long period of iteration and maintenance.