You might say, but the site knows their secret! What if they played the game THEMSELVES? They'd knew what PLAYER secret to use to win! But the only point of that is you're SCAMMING THE GAME. However if 10% of the "game" is owned by outside investors, then you can scam them. So yeah, satoshidice can scam their shareholders but not you.
What stops people from paralleling the SHA256 hashing on many ALUs, like GPUs for example and bruteforcing the key?
The fact that doing so will probably take anywhere between 175 and 24395872948732987 years to complete.
Hashing is a wonderful tool that solves all sorts of problems that when solved make a provably fair casino possible. I invite you to learn as much as you can on the topic.