The glib answer is that cops are lazy and don't want to do their job. Another is that even though the data is there and theoretically could be searched as you

The glib answer is that cops are lazy and don't want to do their job. Another is that even though the data is there and theoretically could be searched as you describe, nobody has built a system that a typical detective could reasonably use to actually conduct that search. Also, professional car thieves would just start swapping plates if the cops started doing this.