The fundamental problem here is the impracticality of proving a negative. In order to conclusively demonstrate that there was no fraud of any kind we would need to have an audit trail from citizenship being granted (largely at birth) through votes being tabulated. That's impossible. So we have standards for what things we investigate and how we investigate them. Those standards have been surpassed out of an abundance of caution in virtually all cases this election cycle.
As an aside, it's interesting to me that the group which normally screams the loudest about government inefficiency and the waste that comes from the government are suddenly the people screaming that everything needs to be checked and rechecked at huge cost to the tax payer when there is very, very little chance of anything changing. This is going to be in textbooks in the future under the heading "Loss Aversion", which I guess means at least we'll get something out of the money we're lighting on fire?
But ... from my understanding, a hand-count is not the same as an electoral audit. An audit exhaustively looks at the systems, procedures, and the path that the data has traveled in order to verify the result.
From what I'm aware of, that is what Trump people are disputing? Please correct me if I'm wrong on this.