Think about all of the left-wing voters in a very Red congressional district or state. Think of all the right wing voters in a very Blue district or state. They are not officially disenfranchised, but their vote doesn't really matter either, so why vote at all? The equation changes somewhat for swing states/districts, but even then it's often a lesser of two evils choice, rather than any major progression towards policy goals. There are some democracies that mitigate some of these issues (getting rid of the electoral college and first past the post would go a long way in the US), but in general democracy leaves a lot of people dissatisfied.
If your major disagreement with the status quo is the tax rate, or certain business regulations, your dissatisfaction with the democratic process is manageable. If your major disagreement with the status quo is police brutality and injustice that make your life miserable, then what is the downside to rioting and destruction? Maybe there's only a tiny chance that something good will come out of it, but it's better than a zero percent chance of enacting change by continuing to vote and lose elections.