One thing to think about here is that US electoral system practically counts different peoples votes differently. A voter in Georgia mattered a lot more than a voter in California or Wyoming. Small polling errors in either state are inconsequential, but hugely consequential in Arizona, Georgia, North Carolina, etc. Lots of polls are national polls, not state polls too. As it happens, the average of national polls predicted the vote totals overall very well. But that doesn't matter. Because it still came down to a few thousands of voters in a handful of states.