Spelling is more a gauge of how attentive you were in early schooling than of how intelligent you are. It's basically a form of conspicuous consumption of the scarce resources of childhood attention and teaching time.

The cultural notion that bad spelling is an indicator of stupidity is self-reinforcing, though: it prevents English from undergoing spelling reforms like those German, Spanish, Russian, and many other languages have had, because any "reformed" spelling will necessarily look like ignorant spelling.

Because English spelling is unusually difficult, it is a challenge. Because it is a challenge, people who have mastered it care about the fact that they have mastered it. And because of that, it can't be made easier.