Considering that some people born without arms are capable of living fulfilling lives with legs and feet only, to serve the purposes that most people’s hands can serve, and considering also the mischief that many people get into by using their hands and arms for “bad purposes”, would you also suggest that we should be born without arms, or that they should be amputated at birth, because they are clearly redundant and get some people in trouble?
As @ucme has correctly noted, the higher the power available to the vehicle’s engine, the faster it can accelerate and (though he didn’t note it) the longer it will last in service at lower “normal” power output. An engine that tops out at 65 mph, for example, at its extreme high end, will be working itself to death every time you move onto the interstate. That vehicle simply wouldn’t last long enough to be economically viable to most consumers.
In addition, the car’s manufacturer does not pigeonhole every driver as to the roads he will use or the conditions under which he will operate. Some people – gasp! – race their cars. (I’m not condoning those who race on public thoroughfares, but a large number of people race, quite legally, on private tracks.)
Some people use their vehicles to tow trailers. The power that could make the car “run like the wind” can also be geared down through the drive train to provide the power needed to pull the heavier load.
Not all cars are destined to spend their entire useful lives on the roads of the market where they were sold.
Finally, even though the speedometer on most cars runs upward of 140 mph, which is faster than most people could drive even if they tried it, it’s unlikely that the as-delivered vehicle itself will be capable of such speed. Have you ever tried to top out the vehicle that you drive, for example? Most consumer-grade tires are not rated much over 90 mph. (Which still provides some measure of safety; it’s not as if your tires will all blow out when you hit 95 mph.) Even if the tires hold up, the vehicle itself may not actually achieve the speed that the speedometer shows as a maximum; the engine may simply not have as much power as you believe it does (to warrant a speedometer that shows a 140 mph top end), or the various systems and vibration of the vehicle would simply not be able to maintain that speed.