Because they don't need them, that's why.
Even if you're driving one of those hand-crafted Italian machines, the ones that look like a jet fighter without any wings and have names nobody can pronounce properly, the police can still catch you. That's because no matter how fast your car is, it can't outrun a radio signal.
And if you're talking about 'factory' sports cars, like a Corvette or Mustang or a 370Z, it's because they're not as fast as you think. Contrary to what every teenage boy will tell you, the average factory-made sports car isn't a whole hell of a lot faster than the average compact sedan. It will accelerate faster, the exhaust system sounds meaner and the body design is definitely much sexier, but those cars don't actually perform any better than anything else.
Now if the police were smart, they would put chrome on everything under the hood of their cars. Because it's a cold hard scientific fact that chrome adds horsepower.