Multitude of reasons, but it's definitely not JUST environmental. For those of us who live in the congestion of Phx, its:
1) Landscapers driving around doing their "xeroscaping" work with rocks constantly falling out of the back of their beds and onto the freeway (this is the #1 cause).
2) Continuous construction of housing, apartments, commercial real estate, freeway expansion, etc, with gravel being transported in large trucks to these construction sites on all of the in-city freeways, dropping gravel and stones everywhere.
3) Lifted trucks/Jeeps that go out into the desert and bring all the small rocks/gravel into the city and leave them on the freeways
4) Lack of rain to clean it all out
Once I'm out of the city, such as day trip to Payson, Flagstaff, Tucson, definitely not as many rocks/gravel. I've decided we can't have nice cars in Phx unless you only use it on the weekends. This will be my last expensive daily driver.