A big part of it is reporting requirements, despite what some people think different portions of the government have different requirements. The reason a person gets inquiries about Venmo payments is because the IRS requires corporations to report that information to them and it's a mainly automated process.

Regulatory reporting requirements of Corporations to the IRS can't be copy pasted onto Pentagon spending reconciliation.

A better comparison, that is still not accurate, would be a cash business that has to report income to the IRS and has trouble getting granular details organized.

Complaints about transparency and accountability on the Pentagon budget are a valid complaint, but this is a poor example.