It's because Android has to scale across such a wide range of device types and sizes. If you look closely at Material Design, you'll notice that pretty much everything can be implemented with vector graphics. The primary colors serve a similar purpose. Not every phone has the ability to display high resolution images, but even the lowest of lowest-end Android phones can display "red" and "blue". The same thing applies to the information density. Displaying information at a comfortable density on something like a Nexus 4 means that the same information displayed on a Nexus 6P (or Samsung Galaxy S7 or what-have-you) is going to look blown-up and sparse. It's even worse when you go to tablets.

I think Google has really dropped the ball when it comes to designing their own apps. Rather than putting in the time and effort to make apps that scale properly (e.g. displaying different amounts of information on different screen sizes) they took the lazy way out and just scaled up the white-space between list and UI elements. And of course, Android application developers are following Google's lead in this regard. This is why there are so few applications that are designed for Android tablets. Whereas Apple's iPad apps look and work substantially differently from their iPhone counterparts (as you'd expect), Google apps on Android tablets look like scaled up and blown out versions of their phone counterparts, with the attendant losses in usability.