I can only think it's because formula feeding on demand is extremely wasteful and basically a complete nightmare. I know because I've done it. And you don't have to build a supply of formula - you just go to Tesco as and when needed.
Isn't it because the make up of breast milk changes - so if baby drinks more and more they get more waterly milk (or something?) Whereas formula milk is the same from start to finish so you can overfeed?
surely the amount you feed a newborn at the time though is minimal? just pour out 1-2oz at a time (it used to be oz when i last bottlefed a baby, not sure what they measure it by now?), as the hospital say u have to bring in your own milk and bottles.
Funnily enough, when I had Alf and was BF, I asked the MW how often he would need feeding. Instead of telling me anything about demand feeding or hunger cues (to be fair, she did say to watch for hand chomping) she said about every four hours . So of course, he was yelling his head off and as it had only been an hour since his least feed it diddnt even enter my head that he was hungery
I was told to feed on demand with mine (formula fed from around 2 weeks) so as long as it was at least 2 hours since the last feed (in the newborn days) then I would offer a feed. It then spaced out to 3 hourly till around 4 months where it became roughly 4 hourly.