Here's why:
* Rock Climb was introduced in Generation IV (Diamond, Pearl, and Platinum). Ruby is a Generation III game (Ruby, Sapphire, and Emerald).
* HMs were used to access areas and solve puzzles. Rock Climb was designed to help players navigate certain areas by climbing vertical surfaces.
If you are looking to explore vertical areas in Ruby, you may need to find an alternative way to do so, such as using a different move or finding a hidden path.