One might assume that ballots and vote counts come close to an absolute truth, that large-scale election fraud is nearly impossible in stable and wealthy democratic countries: Public observers and reviews, recounts and challenge in courts, if demanded.
This refers to the voting and vote count itself, not other influences, like biased media, voter suppression before ballots are cast, or distortion by the process (intermediate instances like districts with "winner takes it all" principle, or the electoral college in the USA).
On the other hand, in less stable countries or fake democracies, election fraud is common: Ballots being destroyed or not counted, fake ballots, minors, dead or non-existing people "voting", or simply fabricated results.
What makes, for example, a huge part of US voters seriously believe that the voting process itself was fraudulent, similar to that of a developing country? I assume that a majority of them does not intend to bring Trump back into office at all cost, even overthrowing democracy itself.
- Trust in an individual (e.g. Trump) over trust in a system
- Confusion of "hard" facts (poll results) with "soft" points-of-view (e.g. biased media).
- Large scale delusion (belief in conspiracy theories, QAnon etc. of entities exercising even unlikely levels of control)