This simple answer is that polls and pollsters are there to make money. They are there to sway the vote not reflect it. There is an entire industry devoted to politics, not governing but politics and they get filthy rich win or lose.
You’ll also notice as the elections near the end the polls tighten. That’s because they are often never really that far apart and it gives the pollster that ability to say, “look that’s what I predicted. It was within the margin of error.” They are a scam as much of politics is.