Because its not as simple as them holding office for one term and fixing everything.
The dems are awful. But even if they had the best intentions it would take decades to un-fuck the situation Republicans have created with court capture and obstructionism.
Since 2016, the Dems have been absolutely terrible. We can blame that on establishment figures like the Clinton family. They simply say elect me, I'm not a fascist, but I won't make any fundamental changes. That's why they lost election after election to literal fascists.