Because it lends legitimacy to the idea that the election has been stolen. The people calling for the audit are exactly the type of people who we need out of Washington: arrogant fools who will adamantly deny reality even if it’s shining in their faces. By giving in to them and performing an audit, it will allow them to waste valuable time and resources, not to mention they’ll probably pick up a few facts out of context and use it to convince fools of their legitimacy.
“If you have nothing to hide, why oppose?” can almost be the motto of totalitarian regimes everywhere. It can be used to justify almost anything. And it’s a terrible idea to base a pointless audit on.