I'm not a Democrat and don't believe in party over principle. You're right, those things you are talking about aren't happening, but some key factors emerge:

1) The Dems are less a party than a coalition, so getting their act together has always been difficult.

2) Because the GOP has staked itself strictly to the far right, the Democrats have to represent everyone from the left to the center-right. If the GOP were halfway centrist, the Democrats could take a more coherent position and have a better chance of passing things. Biden has always been a center-right politician. But the Black vote was scared of Trump and put him over the top. They were probably right, because...

3) the US is not as conservative as the GOP but is still the most conservative (and religious) country among the world's most powerful nations. Getting a genuinely liberal agenda through is likely impossible. We could be a better country, but not full-on lefty.

4) Last and most important: while the Dems have not delivered on a lot of liberal positions, the alternative is letting the Republicans hold power, and they will do everything they can to grind every liberal and centrist priority to dust. Had every non-conservative American voted Democratic for the last 20 years, the Congress would be functional, and...

5) Clarence Thomas would be the only Republican appointee on the Supreme Court. Imagine that.

Letting the perfect be the enemy of the not terrible is poor strategy.