Because every other "generation" is about 10 years and yet somehow "Millennials" are an almost 25 year gap. Notice how it's "Older Millennial, younger millennial, etc". You don't use those qualifiers with the other generations because they are appropriately sized.

Millennials should be 2-3 named generations. It currently refers to 80's kids, 90s kids, any kids alive when 2000 happened, and early Aughts kids(probably because the last name sucked and no one wanted to use it). Too many generations wanted the claim of "I was the first generation of the new millennium" and everyone co-opted the term even when it didn't traditionally apply(newborns because they were closest to the date as opposed to when their major development occured is part of that stretch)