Since this point keeps coming up…
I’m no expert, but as I see it: Biden was a lot younger and fitter when he was chosen to run. Effectively his bid goes back at least to 2009 when he was Vice president under Obama for 8 years. This gives him experience of the nearly-highest office no one else in the race had - Apart from Trump, who’s actually been president 2016-20
The electorate does not chose the presidential hopefuls and not are the potential presidents chosen from the wider population. I think in theory it’s true that anyone can run, but in practice the only people with the slightest chance of success are those endorsed by big donors and the politically powerful. The process takes years. And a LOT of money.
The Democrats problem now is that they can see Biden’s weakness but there’s almost no time to chose someone else within the constraints of the constitution. And if they don’t have a believable alternative to Biden, they’ve as good as lost the election already.
People need to understand that the US system of government is very different to ours in the UK.