These people actually believe that if you get cancer and die, it was your own fault for not thinking their own health into existence! Do they think people starving to death in drought stricken countries aren't trying to wish food into existence? I think they are. I doubt they're all thinking 'man, I never have food, I bet I'm going to starve'. More likely 'Please god, provide me some food so I will live and prosper'. I doubt that it's not happening just because they're praying to someone else instead of 'the universe'.The will to survive is stronger than the will to get a promotion.
An example of why this theory doesn't work: I'm very optimistic about timekeeping. I always think I've got plenty of time. I EXPECT to get there on time, and get my work done on time. But often, I don't at all! And afterwards I think, why on earth did I believe that I'd get there in time? I should begin my journey earlier next time. My optimist expectations do not come into existence half the time.
This 'Law of Attraction' theory (note how the name is deliberately taken from science to weasel in 'credibility') is a mystification and extrapolation of a psychological theory that actually does work- cognitive behavioural therapy. What you think does affect how you feel- the thought comes first, the emotion comes second- and how you feel affects how other people think of you and treat you. Believe in that and it'll improve your life. But that's as far as it goes. You cannot think things into existence. You can only think with the right attitude that will make such opportunities more likely to present themselves.
The 'Law of Attraction' is deceptively 'self-proving', because anyone who tries it out and doesn't get what they expect will disregard it eventually, and the only people nutty enough to continue believing will be the few who end up with the life they wanted.
Personally I think the whole thing is a bit sick. The emphasis placed on financial success is telling of contemporary American culture. In the UK, it's actually considered rude to talk about how much money you have, because it shouldn't be important. And we're only one little step down from America, our culture's pretty sick too.
I'm an atheist but I'm going to use a religious example. In the gnostic gospel of Thomas, Jesus places great emphasis of how "the kingdom of heaven is within you'. I think at least on one level he meant that happiness is inside you. You need very little external things to be happy. You don't need wealth and all that. All these things are just things we strive for as a means to an end. Why bother with 'the means' when 'the end' is already within you? Happiness is the only thing in the world that is sought after for it's own worth. Everything else is just something we do in the hope of attaining it. And this is the one thing you CAN think into existence.
I'm not saying you shouldn't wish, or pray. It's good for strengthening your resolve, and it makes you feel better when you're otherwise helpless. And it might even work (it did half-work for me uncannily once as a child, although I've since thought of it as probable coincidence). But don't start seeing worse off people than yourself as victims of themselves alone, and don't start wanting things so badly that you spend all your life focusing on the future and forget to stop and smell the flowers.