CSPRNG is better than your hand.

When flipping a coin, you will unwillingly (probably) make one or more of the following mistakes:

1. see that you flipped '1' too many times and decide to write a lie on paper that you flipped '0' somewhere in the middle, because it seems more random.
2. the opposite of (1)
3. get bored in the middle of the process and decide to add some bits by yourself.