1) China, USSR, etc had/have more government involvement in running business but also controlling people’s movement and freedom of expression. That’s a different situation than American politics, because even the most progressive democrats don’t necessarily want the government limiting people’s freedoms so much as forcing rich entities to pay higher taxes and to provide more socialized services.

1b) We don’t really need 1 party. More than 2 parties would be great as well. But really we just need the Republican party to not be obstructionist nor actively harmful to democracy.

2) Democrats have the presidency, which, while having greatly expanded powers over the last 50ish years, is still a relatively weak executive. The Democrats have a narrow majority in the House and Senate; due to parliamentary rules, the narrow majority that they have does not allow them to pass or amend laws as freely as you may think. The Supreme Court, while notionally non-partisan, is filled with a majority of conservative Justices which can dissuade or reverse policy made by more liberal branches of the government (federally, or at state levels).

2b) 6 months is not a long time for governments the size of the US. Barrack Obama likened it to a large ocean-liner: there’s a lot of built up momentum, so it takes a long time to stop it or change direction.