There's also people who are just so sick of it all, all the toxicity and all politicians in general, that they perhaps just don't want to.
However, FYI, here's what happens if you don't - and has the possibility of happening even if you do - but you're cutting the odds of it happening if you vote as early as you can.
First - yes, the 20th amendment to the constitution says that if an election result hasn't been decided by January 20th, following the election year, the President and Vice President have to vacate the office - and the succession rules found in the main constitution comes into play - which means the Speaker of the House would become interim president.
Ordinarily, that would mean that Nancy Pelosi would become interim president - and nobody wants that.
(Well, some people do, but many, including myself - don't. I like that she isn't pushed around easily, and has been around long enough to see through this BS easily, as well as some of her ideas - but Nancy can also be just as vindictively obstructive as Republicans, just to get back at them; and at times, she's more than a little power hungry. Now, I understand that Trump leaves you no choice, he can push people to shoot back, but Nancy was like that before Trump lol. )
HOWEVER - Nancy is also up for re-election in November 2020, too.
And Section I of the 20th amendment says that the terms of Senate and Representatives will end by January 3rd following the election year, and they have to vacate, too, if that person dies, or the election is delayed or undecided.
There is supposedly some provision for a vote in the main constitution itself, for her to stay as interim Speaker/interim president temporarily, until the presidential election results have been decided that I'm not clear on - regarding whether this is also a general vote or congressional - but again, a general vote for her to stay or go might also go undecided, and if it's only congressional vote, you know the Senate would vote her out.
Which would mean then that the Senate Pro Tempore would then become interim president, if he or she is also not up for re-election.
And he isn't - that's Senator Chuck Grassley (IA).
I can't think of anything worse than an elderly, even more senile/unstable, over-privileged, angry, out-of-touch-with-the-nation-as-a-whole-and-reality version of Trump being interim president, other than Trump himself still being in office and this nightmare in the streets will continue to escalate.
And you just know there'd be fights over this and lawsuits to the SCOTUS regardless, complicating things further.
Thus, if you vote as early as you can - whatever happens to your vote - you at least did what you could to prevent any of this from happening - right? :)