Because your brain uses information from the areas around...

Because your brain uses information from the areas around the blind spot to make a reasonable guess about what the blind spot would see if only it weren't blind, and then your brain fills in the scene with this information. That's right, it invents things, creates things, makes stuff up! It doesn't consult you about this, doesn't seek your approval. It just makes its best guess about the nature of the missing information and proceeds to fill in the scene...