Why is it that we are only able to access 80% of our brain and not a full 100%

No-one knows for sure. A popular theory has it that the journalist Lowell Thomas helped spread the myth in his preface to Dale Carnegie's block-buster self-help book How to Win Friends and Influence People. Thomas misquoted the brilliant American psychologist William James as saying that the average person specifically "develops only 10 percent of his latent mental ability." In fact James had referred more vaguely to our "latent mental energy." Others have claimed that Einstein attributed his intellectual giftedness to being able to use more than 10 percent of his brain, but this is itself a myth. Another possible source of the 10 percent myth is neurosurgeon Wilder Penfield's discovery in the 1930s of "silent cortex" - brain areas that appeared to have no function when he stimulated them with electricity. We know today that these areas are functional.

I thought that was a myth and wasn’t true at all... oop

The bystander effect  for some reason my history teacher was talking about this