“why have election officials pushed back so hard against a thorough audit?”
Nobody is pushing back. Georgia has already done a hand recount, a machine recount, and a signature audit. Wisconsin did a probably unconstitutional recount in two counties at the demand of Trump, who refused to pay for the rest of the state being recounted because the real purpose of all this is his grifting. He could have asked for a recount in Nevada but again that would have cost him money so none was requested. Trump is supposedly a billionaire but he doesn’t believe in his own cause enough to pay for recounts.
In all other states, Biden’s margin was so large as to make recounts illegal under state law. That is how big Biden’s win is. There is no provision in any state law for anything else to be done, not that Trump and his cult ever cared about the law (as evidenced by his phone call this past Saturday).