Interestingly Venusaur didn't get Earthquake until Gen III.Any time gen 1-2(and to some degree, 3) pokemon's movesets are involved, I wouldn't ask too many questions.
Many of them suffer from messed up early movesets where pokemon learned moves "because yes" as there wasn't much thought behind it and it was transferred over.
Earthquake being learnable by almost everything under the sun isn't exactly much different from the insane amount of early gens pokemon learning Surf or Icebeam or Flamethrower
In fact many older Pokemon that couldn't learn Earthquake in any way in Gen I and II suddenly got it via TM in Gen III. Besides Venusaur, there was Primeape, Hitmonlee, Hitmonchan, Chansey, Pinsir, Dragonite, Mewtwo, Forretress, Dunsparce, Granbull, Mantine, Hitmontop, and Blissey.
Rockslide, Shadow Ball, and Sludge Bomb also weren't available on many Pokemon that were in the first games where they were TMs but could learn them in Gen III. In Sludge Bomb's case, there was a lot of Poison types that couldn't learn the the TM in Gen II, which once again included Venusaur.
While not as wide-spread as the physical moves, some special moves like Ice Beam also got new learners among the Gen I and II Pokemon in Gen III as well.
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