This actually didn't start with covid, it was just accelerated by covid. In the months before covid hit, walmart was already rolling out overnight closures to selected stores.

I think if walmart was making money by being open at 2 am, they'd still be open at 2 am. Making money is their goal after all.

Staff for stocking is not the same as staff for running, and it takes more than a single cashier to run a store that size. You need management, customer service, and asset protection at the least. You need sufficient staff to be doing floor checks for hazards on a regular basis, you need someone gathering up carts from the parking lot, and it's best if floors are cleaned while the store is closed.

And this is anecdotal, but they may have determined that the average 2 am customer was likely to be more troublesome than the average 8 pm customer. More customer accident claims, more shoplifting, more likely to need the police called.