https://en.wikipedia.org/wiki/2024_Democratic_Party_presidential_primaries
https://en.wikipedia.org/wiki/2024_Republican_Party_presidential_primaries
Other people ran in the Democratic Party’s primary. As is typical of US politics, the incumbent president won their primary.
Biden and Trump both won their primaries, because people in their respective parties voted for Biden and Trump. Trump had even more challengers than Biden did. This outcome wasn’t predetermined, just predictable.
Hillary Clinton vs Jeb Bush in 2016 was predictable. But as we saw with Trump’s win, not predetermined.
By electing Biden democracy continues and we will have the opportunity for better choices. There are ways to fix our problems, but we have to be willing to fight for them. If we choose to keep fighting we can avert self-destruction. In this case it means voting for a neo-liberal. That’s a small price to pay for a shot at a better society and averting death camps.