It's not because people aren't voting - it's because our votes are not equal. We don't have "one person, one vote" here - we have "one dollar, one vote", and those with more dollars get more votes. Citizen's United infamously set that in stone. Meanwhile, rampant gerrymandering, court packing, and other anti-democratic tactics have been heavily abused over the past few decades of Republican control to further erode and cement that. Though Republicans are not the only ones accountable - Democrats are just as responsible for accepting PAC money, for instance - and the two party system is mathematically guaranteed to be intractible as a consequence of our voting system. Even if every eligible voter voted in every election, the two party system is an inevitable conseqeunce of any election outcome, and both parties are dogs of the rich.

That the average American has approximately zero influence in politics is a mathematical, deliberately orchestrated truth.