The problem is that the front-end is 100x harder to test than the back-end. Webpagetest, and others, have a really hard time measuring single-page app front-end performance. The front-end is just such a huge patchwork of...stuff...that it is almost impossible to create generic tests, let alone automate them. Otherwise I agree, the front-end performance often contributes more to the UX than the back-end performance. But it's wise to start with the low hanging fruit and test those things that are simple to test. In my book, REST API end points is a good place to begin. Easy to test, easy to automate the testing.