Here's why:
* No Clear Definition of "Boulder": The size and weight of a "boulder" can vary wildly. A small boulder might be manageable even for a low-level Gurdurr, while a massive one would require a much stronger, higher-level Pokémon.
* Individual Differences: Pokémon stats, even within the same species, vary. A Gurdurr with high Strength and Attack stats might be able to move boulders at a lower level than one with lower stats.
* Game Mechanics: The "Move" action in the Pokémon games isn't directly tied to a Pokémon's level. It's based on the move's power, the user's stats, and the target's defenses.
In Conclusion:
Instead of a specific level, a Gurdurr's ability to move boulders is more dependent on its individual Strength stat and its potential use of moves like "Rock Throw" or "Strength".