Why Are Generations So Short These Days?

The alphabet beyond Z, according to Dr. Seuss."On Beyond Zebra"

Hmmm:

Here’s your generational cheat sheet.
As of today:

Gen Alpha: 1-6 years old
Gen Z: 7-22 years old
Millennials: 23-38 years old
Gen X: 39-54 years old
Boomers: 55-73 years old
Silent Gen: 74+

— Jack Appleby (@JuiceboxCA) March 25, 2019

If this is right, the two oldest generations span about 18 years each. The next three generations span only 15 years. Is this really the case? Do we consider 15 years an entire generation these days? Even 18 years seems a little thin to me, so this strikes me as odd.

Also: I don’t approve of Generation Alpha. It should, of course, be Generation Yuzz.