Testing web UIs has always been a difficult job. From unit to end-to-end tests, the fact that the interfaces depend on browsers, user interactions, and many other variables makes it difficult to implement an effective testing strategy.
If you've ever tried to write end-to-end tests for the web, you'll know how complex it is to get consistent results and how the results are often affected by false negatives due to different factors, such as the network. Other than that, user interfaces are frequently updated to improve the experience, maximize conversions, or simply add new features.
If tests are hard to write and maintain, developers are less prone to cover their applications. On the other hand, tests are pretty important because they make developers more confident with their code, which is reflected in speed and quality. If a piece of code is well tested (and the tests are well written), developers can be sure that it works and is ready...