It’s easy coming up with the first version of a web app, especially if you have a mockup. There’s a lot of css and JS frameworks because of how common the use cases are and how easy it is to start solving them. It’s the iteration that sucks. Browser mismatch, difference between mobile and desktops, tools and libraries deprecation,… that’s why you take lot of care in the beginning so you don’t end up in a tar pit.