why the sports car have their engine back?

More often the high end/exotic cars have it in the middle, not the rear (Most Porsches are an exception) This provides better weight distribution and a reduced polar moment (More of the weight is near the center of the car). This improves stability and handling. Also It's my observation that this reduces weight by taking out torque tube/drive shafts out of the mix, as well as bundling the transmission and diff into a single unit, transaxle. Less cases and less parts means less weight, but like I said that's my observation and I don't think I've seen that published anywhere... (I may be wrong)