Why do brands like Gerber routinely sell items containing foods inappropriate for children before 1 yr of age?

Gerber, Beech Nut, Heinz all sell jarred baby foods containing blueberries, strawberries, guava, and sugars not appropriate for children under the age of 1 year. Why do parents trust these companies more than their own pediatrician/nurses? What age did you introduce stage 1, stage 2, and stage 3 foods to your child? What are some things nurses/pediatricians can do to get more parents to listen to the recommendations of introduction of foods into the diet of a baby?