Well . . . either because corrupt leftist politicians and officials colluded with equally corrupt RINOs in order to CHEAT Biden into the presidency . . . with the cooperation of judges willing to call overwhelming evidence NO EVIDENCE.
Or because a significant number of outright STUPID political independents got convinced by the Left and their fanatically leftist loyal Mainstream Media outlets to be terrified of Trump's Mean Tweets; and so they knowingly cast their votes for a blatantly obvious dementia patient.
It pretty much depends on if one really believes that Joe Biden honestly got 81 million votes in 2020 whether one in turn believes mostly in scenario one or in scenario two.