Because they are a waste of space, money, and energy?
In over forty five years of driving I have never changed a wheel beside the road. I have had two punctures and one blowout in that time. The first two were close enough to home to just drive home slowly and then drive to the nearest tyre fitter for a replacement and the last happened at night on a narrow sloping country road where jacking up the car would have been impossible even if I had been willing to risk being run down on a road barely wide enough for two cars.
In over forty five years of driving I have never changed a wheel beside the road. I have had two punctures and one blowout in that time. The first two were close enough to home to just drive home slowly and then drive to the nearest tyre fitter for a replacement and the last happened at night on a narrow sloping country road where jacking up the car would have been impossible even if I had been willing to risk being run down on a road barely wide enough for two cars.