As crazy as it sounds, many per diems were started because 1/3 of travellers would far, far exceed even that amount with the restaurants, incidentals and entertaining.
Yes, some companies now have a generous per diem to add a bit of incentive, especially when rule restricted pay increases overall, but really, they are a net cost saver to the company. Really!
One way the save costs is on the accounting for tracking expenses. Paying you 1 hour to submit expenses, then others 1 hour plus admin to pay them out is quite costly.
Also, if you travel a lot to different sorts of places, you will see that the per diem barely covers some parts of the country (urban centers, Chicago, DC, San Francisco) where you need to pay for taxi's and food, and you will eventually have incidentals, such as buying a book (because there is no library you can use), getting clothing washed (because you have no access or time while working to find a laundromat), even the need to buy a small cooler to fit your grocery bought food into.. shampoo and hairbush when you bag is misplaced for 3 days...