There is activism and propaganda. There is also counter-activism and counter-propaganda, and they're pretty effective. Tons of people don't accept that climate change is real (or even if real, not our responsibility), and they have allies who may accept that it's a problem but far from their top priority.

The result is enough votes in enough places to stop any political solution. Even if there's a majority, the rules are designed to prevent action unless the majority is overwhelming.

People and up doing something, even though they suspect it's not very effective. It at least diminishes the accusation of hypocrisy, that they're not taking it seriously -- not that it matters, since the counter-propaganda will leap on any instance of hypocrisy as proof that the entire movement is wrong. Check any HN argument on climate change, and wait to see how long it take for Al Gore's jet to come up.

It's a poor argument, but it's sufficient to keep the US from overcoming its designed-in bias for inaction.