Some people are genuinely apathetic or feel like it doesn't directly impact their life, but a lot of people fall for the propaganda of "both sides are the same" and that it makes no difference either way, and a lot of people are intentionally disenfranchised by various voter suppression efforts by Republicans. Then there's the electoral college nonsense which leaves the populace of 43 states with essentially no say in who the president is, leading some to wonder why they should bother, not being mindful that their vote may carry weight for the federal legislature and state/local elections. And many people are just too busy surviving to worry about anything else.

For my part, voting straight Democrat in a heavily Republican-leaning state, my vote literally means nothing at all because my state will inevitably give all of its electoral college votes to Trump, and will elect nothing but Republicans to the federal legislature and for almost all state/local offices. But I voted on the first day of early voting, and I will vote in every election, because we have to show support for change if we ever want there to be change. There are enough left-leaning people in my state for it to be a swing state (hell, we had a Democrat for governor 2003-2011, and he was popular), but so many see their votes as meaningless simply because their fellow left-leaners also aren't voting...