The good designers, yes. I'm sure there are plenty of other designers who think their opinion is better than the likely thousands of person-years of UX research that the major OS vendors (combined) have invested into their interface guidelines.
For each of the major platforms, following the guidelines will at least make your app consistent with the other apps on the user's device, which is a decent-enough baseline. Going wild and re-inventing how a drop-down should look or how scrolling should work is probably just going to annoy the user.
I just think it looks ugly, and I don't want to create a product that I don't personally like. Totally open to accessibility and using existing frameworks but not Material.
For each of the major platforms, following the guidelines will at least make your app consistent with the other apps on the user's device, which is a decent-enough baseline. Going wild and re-inventing how a drop-down should look or how scrolling should work is probably just going to annoy the user.