The bare minimum job requirement to being a politician is getting elected. If democrats can’t nail messaging like republicans then that’s on them, not on the average voter. Inflation didn’t really ramp up until trump left office and even though his actions contributed to this ramp up, the effects weren’t felt in full force until after he left. So what happens if you run on “we’re not trump”? People think back to the trump years, and other than when covid hit (which people can excuse as not the fault of the president), the economy seemed to be doing great, and there certainly wasn’t as big of an inflation problem as now.
Also, democrats had multiple candidates running in 2020 who weren’t “another old white dude” but they asked all the candidates they could to clear the way for biden because they needed to focus their efforts to defeat their most feared enemy: Bernie Sanders.