There is a WHOLE lot more to McDonald’s fries than cooking!
[“It was Kroc, back in the fifties, who sent out field men, armed with hydrometers, to make sure that all his suppliers were producing potatoes in the optimal solids range of twenty to twenty-three per cent.”]…
Probably the amount of sheer time it takes versus the convenience and consistency of par-fried frozen fries is the determining factor.
I worked at a Chili’s back in about 1989 and we had probably 5-6 cooks showing up at about 8 am to do prep work. We got boxes of potatoes each morning, and the busboy would wash some specified quantity, and feed them through the chopper (wall-mount fry cutter like at In-n-Out, FWIW). He’d then rinse them in water, and then send them on to the cooks. They’d pre-fry them at some set temp for some set time, and then schlep them into the walk-in cooler to await service later in the day. Meanwhile, other cooks were frying corn tortillas into taco shells, taco salad bowls, and tortilla chips. They’d also be trimming and marinating chicken breasts for grilling (salads and sandwiches). A couple days a week they’d have a larger task- they’d make a huge 55 gallon cauldron of chili, or smoke a massive quantity of pork ribs, for example.
About the only thing they didn’t do there was grind their own beef- we got fresh burger patties each morning. Pretty much everything else was done from nearly scratch.
If the chain went to frozen fries, and pre-made tortilla chips, taco shells and taco salad bowls, they’d cut out 2/3 of the ongoing prep work with very little in the way of quality decrease. I think they still fry their own chips, but they do use frozen fries- they started that like 2 years after I quit (a manager was a friend’s dad and he was lamenting that change one night when I was at my buddy’s house).
Fresh fries cost more. The frozen fries have been pre-cooked to some degree and are controlled for water content. They are produced in massive quantities with automation, the waste is reused and resold. They are also frozen, fresh cut potatoes will not last long. The prep time, waste, and additional cooking time needed for fresh fries is impractical for most restaurants serving large amounts of fries.
Bar none, the best article, with analysis of chemistry and previous work and home prep, of “the McDonald’s French fry,” and on cooking French fries in general: it’s all about sugar (get rid of it with boiling water; add vinegar to water (reduce pectin); freeze (so crystallization breaks the cell structure); refry (won’t blacken due to loss of sugar, and cooks faster and dry interior because water is already released from cells and evaporates out quicker).