Mage spends decades learning magic from the best masters on Azeroth.
Shaman talks to some Rocks 3 minutes after being told about Shamanism from an old guy.
Mages have always gotten the short end of the stick when it comes to amount of effort needed to realize their powers, lore-wise.
(but in fairness they tend to be the most obscenely powerful lore-wise as well, that’s just not reflected in gameplay for the most part).