Because it is also travelling at 70mph, and so is the air in the car. If you want to prove this, try it with a piece of paper in an open topped car!!!
Not very scientific but dramatic enough to be conclusive.
Actually, an even better question - which illuminates why your question doesn't matter - would be: How come if we jump straight up, we land in the same place, seeing as the Earth is moving at 1670 kph (at the equator)? As the others have explained, it's becuase everything experiences the Earth's rotation.
If you drop the coin and then accelerate extremely hard it will move backwards a bit, just like you get pressed into your seat when you accelerate hard, suspect most cars would be incapable of accelerating fast enough to send it through the window :-)