That’s because material ui is a guideline and not set in stone. If you want material UI designs then specify so and designers will give you material design screens.
Discussion
I always specify. It doesn't really work for the average designer. Right now, I require designers to be full-time Android users to help any of my projects in Android. It significantly reduces the pool of talent, but It's the only way to avoid lots of unnecessary debates on UI concepts.
Kind a similar with engineers: don't hire C++ devs for interface work or you are going to have a bad time *on average*.
Def helps to be a frequent user