but we can't make V8 400+ hp engines that get can 40+ mpg's
We can make those, it's all about the package as a whole. You can have the most fuel efficient engine in the world, if you stick it in a 4000lb beast it's not going to get good mileage. That's where the laws of physics come in.
Put that same V8 in a 2500lb aerodinamic car and you'll get significantly better mileage.
So it's not that we can't build the engine capable of that, it's that everything else around it needs to be up to par. And when there are ever increasing safety regulations and everyone wants big wheels/brakes and lots of technology in the car it's going to be tough to keep the weight down.
The reason microchips can be so inexpensive is because the raw materials are very small (think about how much a processor weighs). But when you need 3000lbs of carbon fiber to make a light car well that much material is going to cost a lot.