But why not 3 years?
As we all know, let's say if we are given an assignment for college to do and we are given a week to complete it, almost always, I will start the assignment about 3/4 days into the week and then as the deadline looms closer, the real work happens! So in answer to your question, if I said 10 years, I would be focused on 10 years :)
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".