Why do tornadoes only occur in the US?

They don't only occur in the U.S. Tornadoes have been recorded on every continent except Antarctica and some of the deadliest tornadoes have occurred in Asia. However, the U.S. does have the highest number of tornadoes of any other country. This is due mostly to the climate setup, which has a near perfect setup for combining colliding air masses, instability, and strong wind shear to create strong, rotating thunderstorms called supercells. These storms produce the vast majority of tornadoes. Additionally, the U.S. keeps detailed records of its tornadoes, something most countries don't do.