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 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).
You’re further proving my point. A person born in 1981 would be 18 years old in 1999. They will have had NONE of their childhood during the Millennium(unless you’re counting the very end of it)