I've been asking for weeks why if there was a "better" or different Democratic candidate than President Biden then why didn't they step out in 2021-2023? It's quite late in the game to change horses.

That’s an easy one to answer. The DNC endorsed Biden early on (January 2023). They also rubber-stamped a primary calendar that was favorable to him, and stated they would not sponsor debates. So primary challenges were discouraged, despite polling indicating voters favored a new candidate.