In my view, it's mostly because the Democrats are a lot more diverse than most people think. They range from traditional conservatives to (relatively in America at least) stout progressives. The Republicans have the power of religion and a very uneducated voter base. People expect more from the Democrats since the Democrats represent a wider range of overall people and higher educated people than what the Republicans represent.

People want to be represented but the Democrats have too wide of a policy window to truly represent everyone so people complain that the Democrats aren't doing what they want because they realistically can't due to how wide they are on the political spectrum. The first past the post system only allows two parties to have a chance of winning. The Republicans represent the right wing extreme which are (usually, maybe not anymore after Trump) unified in what they want and will do.

It doesn't help that the Republicans constantly do their best to block whatever the Democrats do and then have the media pin the lack of getting shit done on the Democrats rather than the Republicans constantly fighting back everything the Democrats do. It also doesn't help that since the education system in America is barely the bare minimum, so many people are uneducated and unwilling or unable to learn what's really happening in the political space so they believe what their told from the people around them like their family, neighbors, or friends.

America is very polarized and all forms of media are just pouring gasoline on the fire. It'll be a hard issue to fix but what we need to do is get rid of the first past the post voting system and the electoral college and then the issues will slowly resolve themselves over the years.