Fraud happened.
It almost always happens. It's been happening since the first vote was cast thousands of years ago.
The game is almost always rigged in some fashion.
Precious few votes are honest and most of those honest votes belong to very small groups with high transparency, a very small relative wealth and power gap between all the voters, and potentially a few other factors.
The question(s) should be 'how much fraud happened and how obviously?'
The latter half is probably the more important bit.
Voting, especially when there is low transparency on the government's end due to whatever factors (such as the sheer immensity of the nation), very much relies on a level of faith and trust to be legitimate. If you feel you cannot trust the integrity of the count, that there is a lack of faith and trust for whatever reason, you cannot trust the outcome of that count to be legitimate. Which undermines the whole process immensely. If the outcome is not legitimate, most people, all over the political spectrum, believe you do not have to respect it.
So... 'the fact that I could have believed something to be false (Because of things like of computer glitches that were waved off, sudden huge spikes in counts at late hours from the already sketchy and much less secure mail-in votes that scream fraud, the resistance to methods to make voting more secure against fraud, and the blatant attempts at importing a voter base that they can move to areas to weaken their political opponents) says something' is a completely legitimate and not at all stupid argument.
That there are solid reasons to believe, if only circumstantially, that the Democrats (or whatever party is applicable wherever you are) are fucking with count, is a huge problem. And the lefties are convinced it's the Republicans who have the power to manipulate the count thanks to media spin.
Nobody is going to trust this election, they're just going to cling to their candidate as the one who won.