Pacquiao greatest of all time?

Many times when I go to boxing shows around the country I will listen in on conversations and hear writers or fans commenting on various fighters. Something common that I hear are the words "he's one of the greatest of all time" This could be referring to a Roy Jones Jr, or a Larry Holmes, or a Rocky Marciano. It's just a common phrase that you hear people say.
In Manny Pacquiao's case, why isn't it OK to say, "THE greatest of all time?" I think with the accomplishments that Manny Pacquiao has achieved throughout his historic career, winning 7 world titles in 7 different weight classes, this isn't such a crazy notion. Pacquiao very well may have eclipsed from the, "ONE of the greatest of all time" phrase to "THE greatest of all time" phrase.
With wins over Erik Morales, Marco Antonio Barrera, De La Hoya, Hatton , Cotto and Marquez, (all possible hall of famers), why is it so crazy to call Pacquiao, "THE greatest of all time?" The great Muhammad Ali won the heavyweight title 3 times. Pacquiao has won 7 titles in 7 weight classes, an unheard of achievement! So again I ask, is it so crazy to call Pacquiao the best of all time?
Several of Pacquiao's fallen challengers will be in the Hall of Fame one day. That's pretty amazing in itself. Pacquiao destroyed future hall of famers on his way to the top, this is something that we all need to remember.
So, Manny Pacquiao the greatest of all time? Why not?