Marketing.
Aside from the "baby boom," which was an observable sociological event in the post war period, all these other names are nonsense from ad people and the press. "We named one generation, so we have to name them all!" They also use the Baby Boom time range to define other generational cohorts, which makes no sense. Zoomers are like 97 to 2012. A kid born in 97 has a completely different experience than one born in 2012. Its all just branding.
That said, they do make good short hand pejoratives. Some better than others. We never came up with a good short hand to bash Millenials.and nobody cared enough about Gen X to ever bother.