For the same reason that there are a million people in Houston with paper tags that don’t get caught. Unless you’re actively committing a moving violation or the cop just absolutely doesn’t like you (that could range from personal vendetta to driving a car they don’t like to driving while black in a middle class neighborhood), there’s a good chance they don’t notice your plates.

I’m sure, while I think cops are by and large very bad, that even for them it’s above their pay grade to tangle with one of these morons and it’s easier to just look away. Cops have been killed by sovcits so I can’t say I blame them.