Why do most of the world's caves form in limestone?
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Answer: Slightly acidic water slowly dissolves limestone and widens its cracks
Flowing lava hollows out tunnels inside limestone layers — Lava does make caves, but not in limestone. A lava tube forms when the surface of a lava flow cools and hardens while molten rock keeps draining underneath, leaving a hollow tunnel in basalt. Lava-tube caves are far fewer than dissolution caves, and they never form by hollowing out limestone.
Earthquakes crack limestone more easily than other kinds of rock — Earthquakes do crack rock, but a crack is not a cave, and limestone is not unusually easy to shake apart. What makes limestone special is chemistry: it is calcium carbonate, which dissolves in weak acid, so cracks of any origin can be widened into passages over long spans of time.
Slightly acidic water slowly dissolves limestone and widens its cracks ✓ — Rain and soil water pick up carbon dioxide and become mildly acidic. Limestone is calcium carbonate, which that weak acid dissolves, so water seeping along joints and bedding planes removes rock grain by grain. Over hundreds of thousands of years the openings join into passages and chambers; dripping water later builds stalactites and stalagmites from the dissolved mineral.