Biden was incapable of clearly communicating verbally for 2 hours straight in a debate. I’d argue that Biden stayed in as long as he did to enable them to feel empowered to force another Kamala on us rather than having to deal with a Warren, Sanders or even a Buttigieg winning the Primary. Kamala was 6th in line in the Primary when she dropped out in 2020.
In an American vacuum I could see where you are coming from. In comparison with literally the entire rest of the world, it is clearly a flawed standpoint.
The American Democratic party is the oldest standing political party in the entire world. It last changed it’s political stances in the 1960’s and not because they wanted to, but because they needed to respond to the Republicans flipping the entire south in their favor.
Other countries have real leftist parties that actually get government members elected.