That's what I thought, too! I read Immanuel Velikovsky's "Worlds in Collision" decades ago, and ever since then it made sense that a planet breakup formed the asteroid belt.
Actually, most serious astronomers note that during the early history of our solar system, there were actually quite a few more planets than now. But collisions between these smaller planets during that early time not only shaped our own Earth, but possibly Venus, Mars, and the Asteroid Belt between Mars and Jupiter. Mercury may be a case of a planet that while suffering smaller meteor impacts never had an impact with another proto-planet like Earth probably did.