I think it's generally caused by a few issues:

1. UI development is hard to do well. Throughout my career I've seen complexity explosions in C++/Win32, C#/WPF, and JS UIs so this isn't a new phenomenon

2. It's always easy to develop the correct UI in hindsight once all the requirements are known. When starting out it's hard to know what will be needed or not and this leads to overly complex solutions

3. The browser as a platform has historically been painful to use due to a lack of good standard library

4. JS in particular has a low barrier to entry, so a lot of folks start there and learn as they go. This leads to picking the wrong tool for the job due to lack of experience