Infant formula is heavily promoted in the United States and the rest of the world. From the beginning, formula-makers agreed to eschew direct-to-consumer advertising (mainly to make the product seem like a prescription medication—times have certainly changed!). Instead, they came up with other ways to promote their product. Manufacturers lavishly pass out samples to new parents; when it's introduced early, supplemental formula-feeding compromises the success of breast-feeding. Health workers of all kinds receive lifetime supplies of free formula. Hospitals receive free or deeply discounted formula for use in maternity wards. The assumption is that parents will think the product they got in the hospital is the best one, and that once they've started using a formula, it shouldn't be changed (though that's not the case).
Manufacturers also claim that formula closely resembles breast milk. It's true that the nutritional content is regulated by federal agencies. And since all brands must meet the same standard, most doctors consider them essentially interchangeable. (Except for the ones marked "low iron," which means for all practical purposes no iron, and are almost guaranteed to produce anemia). But whether or not it meets nutritional standards, formula is not very similar to breast milk. Granted, it contains similar amounts of protein, fat, calcium, and carbohydrates. And formula and breast milk produce similar rates of infant growth. But the resemblance ends there. Cow milk or soy proteins, the basic ingredients in almost all infant formulas, are just not the same as human proteins. One way in which they differ, for example, is that cow and soy proteins are much more likely to cause allergies in human infants. (I have only once come across a baby who was truly allergic to mother's milk, but it is pretty common to encounter formula allergies.) And formula does not have the antibodies that act locally in the baby's intestine to protect against diarrhea and perhaps other illnesses. In addition, human milk contains many, many minor or trace ingredients whose function we have no idea of, but some of which are almost certainly beneficial.