They don't happen instantly, but they do certainly form quickly compared with other types of storm. This simply has to do with the scale on which they exist. In weather terms tornadoes are microscale weather patterns, referring to things that are less than a few kilometers in size. (The largest known tornado was 4 km, or 2.5 miles wide. Most are much smaller.) Because of their small size the mechanisms behind microscale systems operate quickly, causing them to form and dissipate in minutes or seconds in most cases. In addition to tornadoes, other microscale systems include turbulence and individual cumulus clouds.