Dude is so stubborn.
user
Dude has a chance to leave with grace. If he runs and trump gets elected, he will be remembered the same way RBG is remembered
assistant
In other left subs people talk like with Biden, there's at least a small chance to win, but with other candidate there is none. 

Since I'm not from America, I don't really get, why it is considered " Too late" To build public support for new candidate in half a year. Is campaigning in US so different?
user
The election cycle in the US starts 1.5-2 years before the actual polling day. Keep in mind that terms are only 4 years at a time, so there's basically always a quiet election cycle as different hopeful politicians try to elevate their status and get more popular in anticipation of an election. The Democratic National Convention (where the candidate is formally nominated by the party) starts August 19th. Technically, neither party has an "official" candidate, as the winner of a primary is only nominated at their respective conventions. So even though the election isn't until November, there's really less time for the DNC to gather behind a candidate if Biden were to drop out.

It is incredibly uncommon in the United States for an incumbent president to not run for reelection after their first term. Only 6 presidents have ever not done this, and it hasn't been done since Lyndon Johnson in 1969. *Generally* speaking, the running incumbent wins. In the last 50 years, this hasn't happened only 3 times (Jimmy Carter in 1981, George H.W. Bush in 1993, and Trump in 2020).

Joe Biden and Donald Trump are the most notable and major figures in their parties at the moment. Donald Trump has been the face and leader of the Republican Party for almost a decade, and Joe Biden has been working in politics for most of life in addition to being the current president. The primaries in this election were basically a joke and a formality, as there'd be no way the Republican National Convention didn't nominate the most popular and influential political figure in their history, nor would the DNC forgo Biden, the incumbent who has already beaten Trump once and is the most prominent figure in his party at the moment. If something happened to him (death, stepping down, etc.), I imagine Kamala would be the defacto replacement as a candidate, but she's had almost no major public profile as Vice President. 

I don't know if it's literally unprecedented, but it would be a huge shakeup for the incumbent president to stop running for reelection a month before the convention he'd be nominated at.
assistant