It does leak and it has turned into a massive toxic clouds, but ammonia is much, much lighter than air and a (comparatively) unreactive gas so any large release

It does leak and it has turned into a massive toxic clouds, but ammonia is much, much lighter than air and a (comparatively) unreactive gas so any large release dissipates extremely quickly and rises straight up out of buildings and infrastructure. It doesn't linger and there's no danger of invisible structural embrittlement or fires. It's also extremely noxious at even very low (safe) concentrations so, unlike odorless methane, hydrogen, or carbon monoxide, people know immediately to stay away from the area. The natural gas (mostly methane) that's piped into housing for heating or cooking is deliberately ordorized with hydrogen sulfide to make it noxious so gas leaks can be smelled.