I'm not really interested in arguing this kind of stuff and I don't disagree with you that a lack of voter engagement is a problem. But, I would encourage you to try and understand exactly why it seems like people don't give a shit about the state of politics.
I'd be willing to bet that it's not actually a lack of giving a shit, just a feeling that our time is better spent on other things in life. Those 80 million people "sitting on the sidelines" aren't complaining for the fun of it, they are busy trying to live their lives and deal with their own problems. People feel like the system is rigged, not because of some ambiguous statistics, but because every time they try to work with the system they get shit on and forgotten. How can it not feel rigged when the majority of the country votes for one president and gets a different one instead? Or how about when states, without ever asking its citizens, take away a persons right to choose what happens to their own body? How is a system with an archaic electoral college, gerrymandering, corrupt politicians, and a parties that only represents the top 1% not a rigged system?
It's not that we don't know that showing up in numbers is a good way to enact change, nor are we just sitting on our collective asses complaining and expecting things will just magically change. We just aren't holding out hope that enough numbers will show up to make a dent in our lifetimes, or that the changes will even be ones that benefit us.