Because it’s interesting and entertaining. Kind of like football. The difference being that not even the most diehard Steelers fan really believes that the future of the nation rides on whether their team makes it to the superbowl.
And of course there is always the opportunity to find and connect with fellow nonvoters, and sway others to nonvoting conservatism.
As for your graphic, if you wish to believe that your one vote in one hundred million cast for one of two carefully screened uniparty candidates matters, then please do show up.