This is purely based off of my own experiences, and should be taken as such.
Eh, that's because the educational system would rather students feel good about themselves than having them learn something. In elementary school, we did more coloring than reading (I could read already, since my parents taught me before I started school). In middle school, you could only get held back once; after that, you'd just keep moving up a grade until high school. In high school, a "D" is passing (rather than the usual "A," "B," or "C"), making it much easier for people to earn their diploma without actually learning anything.
Basically, my school district is run by morons who care more about students feeling good about what they've "accomplished" rather than teaching students skills and giving them knowledge that they will need to be successful in life.
EDIT: I should add that, I realize there are exceptions, where life may have just kicked the crap out of someone and they've had horrible luck and didn't have a chance to get a decent education. The above is based on what I've seen from the people at my schools, and the policies that these schools have.
