And trunks? and sunroof plugs? and fenders? I know I've seen plexi hoods, but I doubt their structural integrity. How about wheels with the inside spokes that are clear. hmm....
But see, then you run into the same problems you run into with carbon fiber (not saying plexi is safer). When you get into a wreck with a carbon fiber hood the hood either shears about half way through and you need a new hood, or in some cases it can actually break the hinges and come off the car completely (rare, but since ive seen it happen i figure its probably happened to others).
When you replace an aluminum hood with a stronger alloy it completely throws off the 'crumple zone' and can potentially cause more harm than good. The plexi hoods just shatter when youre in an accident (never seen it happen, but if a basketball player can do it, im sure a car wreck can).