They answer is that they're both funded by and held hostage by entrenched corporate interests. The Democrats in safe blue states are allowed to say and propose whatever lofty reforms they'd like (more fuel for the fire), but when it comes time to vote on a bill, the "moderates" in purple states fall in line to kill it and/or add enough loopholes and pork that the reforms end up pretty hollow. They're also fully supported when they ham up the identity politics and other divisive topics that encourage the plebs to fight one another.

The voting public gets tired of the squabbling and seeing the constructive type of reform fail, and switches its attention to the other professional party. The "reforms" then put forth by Republicans are all about getting rid of restrictions on corporate interests, appealing to a fallacy that restrictions on corpos are akin to restrictions on individuals (from the same vein as Steinbeck's "temporarily embarrassed millionaires"). When the Republicans get the green light, it's then full speed ahead for sidelining the US government in favor of unaccountable corporate power - as we've seen demonstrated in stark relief with Trump.

People will then slowly realize they've been had, and support for Democrats will grow. But any attempt to rebuild what got destroyed then fails (the original dynamic), and the cycle repeats.