Lore wise, Malphite is the size of a literal mountain. Taliyah doesn't have the power to manipulate THAT much rock, and her movements seem more... how to put this...
Her power comes from the Stoneweaver, and her spells/abilities seem to reflect that. They flow, spike and shift. It's less that she has a ton of raw power and more that she can apply it in a specific manner (her stone wall doesn't just shoot up out of the ground, it flows out like a wave). If that makes sense.
Skarner's species isn't fully crystalline, tho. Even if she could control him, I don't see why she would.