Why should all Pokemon learn it? Some Pokemon just don't match the description of the move because they are portrayed as small quite Pokemon.
Work Up Description
The user is roused, and its Attack and Sp. Atk stats increase.
Do you really think a Celebi or any over peacefull Pokemon could use that?
Work Up is for a Pokemon that is naturally violent like Poochyena
It savagely threatens foes with bared fangs. It chases after fleeing targets tenaciously. It turns tail and runs, however, if the foe strikes back.
Therefore I don't think every Pokemon should be able to learn Work Up as the Pokemon has to be naturaly violent.
Hope this helps.