Not ever user runs a water or grass type on every team, nor leads with it everytime. There is a WIDE variety of viable strategies. Solid Rock may be Rhyperiors more common ability, but that doesn't mean it's the only useful one.
Rhyperior has enough physical bulk to not care about Solid Rock honestly. Plus Soild Rock won't save you from Stab Water/Grass moves anyways. Protecting a x4-x6 easily exploitable weakness is more important, especially when a Gyarados at +1 or +2 Atk and Speed can sweep a team, Rhyperior is much less likely to do that.
I personally prefer Garchomp + Zapdos (Equake+Rock Slide, and Discharge/Heat Wave)
Hard to fight fast and strong pokemon, with such reliable dual coverage.