They're just believing what their information-circle is saying. They're not looking at things themselves. For instance, in the discussions I've seen of the Right talking about fraud, they've not mentioned the shenanigans of the USPS not delivering mail that arrived on time (WaPo yesterday had a story IIRC).
They need to believe. So they follow those that help them do so.
The reverse of that is likely just as true; people that are happy Biden won, have no concern if there was fraud, just like those that wanted Trump to win would not question him winning.
You're probably right. I'm not a fan of Trump, and I'm not sure how I'd feel if I found out that there was truly fraud and the election belonged to Trump.
But I am in favor of investigating all (legitimate) allegations of fraud, not just the ones that DJT wants to investigate. And counting all ballots.
This is probably the new normal. Republicans have learned a thing or two of after four years of claims about Russia stealing the election. Both sides will work overtime now to delegitimize winners whenever possible.
They need to believe. So they follow those that help them do so.