You should give The Black Swan a read sometime. Lots of people are smart, well prepared, work hard, but few get the phenomenal returns that Mr. Cuban has. That's luck.
One example is doctors: they are people who work hard to achieve, do an important job, and certainly earn well, but they don't seem to get these stratospheric startup paybacks, just a good, steady high return on their investment in learning and practice.
To me "luck" is like the word "love", the moment you start using it to describe things, you stop thinking about it. Perhaps in the case of love thats a good thing, but when trying to be succesful at something, going around calling things luck instead of trying to find the source does not do you any good.
The fact that we were born in North America and not into a war torn anarchy based country like Somalia, we should all consider ourselves lucky. From that point on what you choose to do with your time and the results of that time spent ain't got nothin to do with luck.