Except that Hillary didn't really have a 14 point lead on Trump at that point, unless you cherry-pick the outlier poll. 538's aggregate polling had her up about 6.5% at that point, and even that was just about the widest margin she had the entire campaign run.
Their aggregate numbers even showed that lead narrowed to lows of 0.2% on 7/30, 1.1% on 9/20, and even 2.7% on 11/4. That lead was far more volatile than Biden's is now. Biden's lead has remained consistent throughout, and has only really broadened as time moves on.
And while Trump was less of a known political entity (at least to the masses) in 2016, everyone now is
painfully familiar with the kind of President he would be with another four years. So the number of people who might vote for him on a lark just to "shake up the system" is probably fairly non-existent at this point.
Also at this point in the 2016 election, there were 19.2% undecided/third party. Right now in the 2020 election, it's 6.3%. Biden's consistently polled over 50%, so Trump can't simply pick up a large chunk of undecided/third party votes in order to surpass Biden. Hillary never aggregately polled over 46%, but ended up with 48.2%. Biden, on the other hand, has never aggregately polled
below 47.5%.
Trump knows all this, which is why he's doing everything he can to suppress voters.