why is this race so close?

If you're posting this right now because of recent poll data, It's because Republican companies are creating their own polls just so that they can claim the race is closer than it is.

Of course we don't know who's going to win. The election hasn't happened yet. But any speculation about changes in the last few days should be taken with a grain of salt, because this is a playbook that was trotted out two years ago and it's back again.