Because they are basically unenforceable. How are you going to make people keep their guns in safes at home? Are you going to have cops do random checks to make sure? Have check points like drunk driving ones to search cars to make sure no "assembled" guns are in there? They going to have set times you can have the gun out of the safe?
The State of Florida requires people to keep guns secured by some kind of lock when they are not being used. If a gun owner's child shoots somebody with a gun left unlocked, then the owner is criminally liable.