The Democratic platform is really quite leftwing. It contains things like increasing the minimum wage, getting money out of politics etc etc. The problem is in how they run campaigns. The role of consultants is far too big, and this lets the GOP set the agenda.
Example: The GOP talks about the border constantly->media reports on the border-> voters in focus groups report caring deeply about the border->Dems campaign on the border (arguably their weakest point).
If Kamala had campaigned heavily on healthcare (say expanding Medicare), she could have shifted part of the focus away from the border and towards healthcare (the GOPs weakest point), which shifts the momentum.
The same happened with many other topics. The campaign talked about the economy (whatever that means, but somehow voters associate this with GOP), rather than raising minimum wages or building homes (a strength of Dems). Climate change was never even mentioned throughout the campaign.
Dems have to find a way to lead the conversation, rather than follow a conversation set by the GOP or they will never win.