Working Hard is only one aspect...working smart is the other aspect.
2 people could work with the same level of steadfastness and intensity for 10 years...and the end of the 10 years, one guy made a pile of lamp posts...the other made a car that runs on water that revolutionizes the industry...they both worked equally hard (in terms of the 'sweat of the brow' metric), however, the latter of the two is definitely going to end up with far more money, and it probably going to be in a position where he can make money without having to work very hard anymore in the very near future... and rightfully so.
...or another way of wording it, rarity.
In the 'sweat of the brow' sense, a McDonald's employee works harder than I do in an average day. They're on their feet, making food over a hot fryer for 8 hours. I sit at a desk, writing .net code for 8 hours in a comfortable chair in my office...yet, I make far more than a McDonald's employee, and here's why: 99% of the population could do that job. If you look at the demographics of software engineers (wiki has a good link that goes into the numbers), only 1 out of every 200 workers has the qualifications to do my job. That makes us more rare, and thus, allows us to demand a higher level of compensation from our employers.
If what I do, or what a CEO does is so easy, then why don't the McDonald's employees scrap their current jobs and start their own companies and start earning the big bucks?