[nandemo]

The analogy doesn't hold. The problem is not so much that your school assignment is 1 million times easier. There's also:

1) There's not such a huge variation in the time necessary to complete an assignment. Your professor knows that some people can finish it in a couple of days but others need a whole week. But it's much harder to estimate the time to get $1M.

2) You can finish a school assignment on your own. Assuming you have access to the necessary resources (books, net, etc) it depends only on you. You don't need any luck to finish it. But to get rich you need favorable market conditions, meeting the right investors, hiring the right people, the government not screwing you up, etc, i.e. "luck".