Because it is a golem. It is what golems do. They're all earthy.
In Pokemon diamond and pearl golem learns earthquake at level 33
Level 49
A Nidoking learns Earth Power at Level 43.
horbddvdgngth
Dialga learns the move Earth Power at Level 60.
It doesnt
Whenever you catch giratina at level 70, it should already know earth power.
74
It cannot learn Lava Plume in Emerald because Lava Plume was not created until Diamond and Pearl came out. It will learn Earthquake at level 37 and Eruption at level 45.
golem numel/camerupt diglett/dugtrio other ground Pokemon
Beast + Life = Human Golem + Life = Human Life + Clay = Golem Lizard + Earth = Beast
It never learns earth power unless you trade it to a 4th generation Pokemon game.