I kind of agree but I think that it could be an internal marketing thing, to create cohorts inside the USA to make it easier to sell every sort of stuff to them. Then it spilled internationally because of the cultural influence of the USA. I'd appreciate if anybody from the USA could tell me if my hypothesis is at least plausible.
According to Wikipedia [1] "The contemporary characterization of these cohorts used in media and advertising borrows, in part, from the Strauss–Howe generational theory" but the chart over there despite being captioned as "Timeline of generations in the Western world" seems to be very USA specific given the definitions in the text that follows it.