Design guidelines are just recommendations. Frequently bad ones. A developer should design the best UI he can, not follow what Google says regardless of whether it fits. And most developer guidelines, Google and Apple both, are crap.
The problem is that the whole app movement has brought in a whole slew of crappy developers who's idea of coding is to search stack overflow or git for stuff to copy paste. They don't read it, don't understand how to use it right, and expect it to magically work. Worse half of the people writing that code fall into the same category, so its the blind reading the blind. If you pick a library off of github and assume it will work, you deserve what you get. Unfortunately your users don't.