There's a number of reasons for this like the electoral college and voter suppression.

The electoral college basically means that all of a states votes go to the winner of that state. If you're casting a minority vote in that state it's very easy for it to not affect the outcome in any way, unlike if the winner was determined by popular vote where every vote mattered. This is also why we see presidents elected even though they receive less votes, which further undermines the system.

Then there is rampant voter suppression, usually in Republican controlled areas. There is a strong correlation between more people voting and Democrats getting elected so Republicans try to make it as difficult as possible for people to vote, especially people who are likely to vote Democrat. So they pass voter ID laws, restrict mail-in voting, close polling places and shorten polling hours. In these areas people might have to spend hours traveling to a polling place only to stand in line for more hours before they can cast their vote.

Add in that voting takes place on a work day when it should probably be a national holiday, the two party system, and other stuff and it's easy for people to feel lukewarm about voting.