Because, humans aren't random number generators. What is randomness? Complete lack of determinism. If something can be accurately predicted, it's not random. A cryptographically secure random number generator comes with more unknown variables to predict, in comparison with a human brain.

Begin writing. What's the first binary value, and why? You might think there isn't a reason you chose 0 (e.g.) but there is quite likely a reason you don't know.