Because that would make way too much sense. You have to remember that we are dealing with government here. Government, at best, is a necessary evil. Therefore the founders made an effort to keep it tied up in the chains of the Constitution so that it only had authority to do a few things, i.e., things mostly having nothing to do with the daily lives, liberties and properties of the people. This was a good plan, but it only lasted till the middle 19th Century. That part of the Union which wished to preserve political liberty attempted to secede from that part which didn't wish to, and the latter folks won. So much for government by the consent of the governed, and so much for strictly limited government. That's why.