I agree with you in certain aspects. Democrats are certainly not left of center by any means. None. Nor are they centrist. They've always been right of center. But sitting next to the far right Republicans makes them appear reasonable. Overton window.
That being said they threw us a bone by pushing biden out, which is inherently progressive (even minimally). The conservative traditional dem way would have been to stick it out with biden. Also, Walz was the most progressive pick of all the VPs. He's no bernie, but it's still something. Way better than the alternatives.
And lastly but most importantly, trump. Trump is the worst of the worst of the worst of ruthless exploitative conservative republican capitalism. He's a cheater, exploiter, and now a convicted felon. He would love to be a fascist dictator IF he had any opportunity. And whether he publicly says so or not, that project 2025 horror is real and what much of his base wants and what his "cabinet" would absolutely push for.