"two thousand and nine" is one more syllable than saying "twenty oh nine"
Why we continue to say two thousand X year makes no sense to me. It's just a thing that made it into the English language and it's been repeated.
Discussion
We were taught to count like that lol
I think for the 2000s decade most people say "two thousand nine" (no wasted syllable on "and") and for the 2010s people tend to say "twenty eleven". When the century is implied it's "oh nine".
It seems to me people generally already use the least amount of syllables that fully specify the year.