Groudon's Sp. atk is pathetic (by uber standards), so Eruption is actually quite useless. Even if you made a Sp. atk set, after he takes a single hit, he no longer hits hard.
Same with Solarbeam, actually. Except it has paltry power compared to Eruption, which is already a weak move.
Groudon isn't as good as you think he is (for uber standards). Being hammered by Ice and Grass, while garnering low Sp. def means he has a weakness for every single team (all uber teams carry multiple pokes with Ice, one or possibly two with Grass). Since a choice scarf set isn't nearly as scary as Kyogre, he doesn't function as well as a revenge killer either. Considering plenty of levitate pokemon can switch in on a banded Earthquake, he can also be made helpless in most situations.
The best Groudon can do is rely solely on prediction and hope to hit hard with a CB, or sweep with Swords Dance; but only with Paralysis support (usually Blissey, they pair well).
Basically, my point is that Groudon's best sets are either risky, or need a lot of support to be pulled off. When he does get things right, he's very scary, but he's not as standard as most other ubers. His primary purpose is simply setting up perma-sunlight.