I think the problem I’d related to the charges on the cards not having oversight for legitimacy. And I know that the govt doesn’t question the thriftiness of employees and their purchases.
That lack of oversight is an artifact of much federal spending independent of the vehicle by which the transaction was made.
On my government credit card, the charges were carefully monitored.
You were allowed, as I recall (more than a decade ago) about $50 per day for three meals.
You had to stay at a government approved motel/hotel, which honored the government rate (usually about $75-$125, depending on the area) very high priced areas had higher rates or you could not get a room. All the transportation costs for air fare were at standard commercial rates. Rental
cars were at an approved, discounted government rate, if you wanted extra insurance, you paid for it yourself.
A few times I was able to use my private car and drive, when I could show it was cheaper than the cost of air fare and a rental car. I think I was reimbursed at about 50 cents a mile.