Because the baby boom that created the boomers was an actual generational cohort that shared a TON of similarities and had a huge impact as being the largest slice of the male population as so many died in the two World Wars. That lead demographers into thinking all generations worked like this and it took time to have other generations to compare them to.

Pew just announced recently they are moving away from generational cohorts because while someone born in 1979 is technically gen x they have a lot more in common with millennials born in 1985 than a fellow gen x er born in 1967