Maybe the OP heard a rumor that they close.
Rain at Walt Disney World and other theme parks can be fun. A lot of people leave and lines get short. We rode Expedition Everest in the rain 3 times in a row one time. It was fun! (Medium to light rain with no lightning) A large percentage of attractions at WDW are indoors.
Most theme and amusement parks do not close in the rain, including Six Flags, I assume you mean that many outdoor rides close. Parks will close rides that are unsafe to operate due to the rain or the possibility of lightning.
There are also extreme circumstances where local extreme weather will cause a park to close (tornadoes, hurricane, extreme heavy rain with flooding) but these are very rare.
From Wikipedia: "Since opening day, Magic Kingdom has been closed temporarily because of five hurricanes:
Floyd,
Frances,
Charley,
Jeanne, and
Wilma. It was also closed from mid afternoon September 11, 2001 to September 12, 2001 due to the
9/11 terrorist attacks."
Some of those closures were for part of the day. Epcot closed once due to a power failure in 2002. It has to be a hurricane direct hit for Disney to close. Several other hurricanes have hit Florida and Disney did not close.