Why doesn't gills work on land?

It is physically impossible for people to grow gills, because God made us this way, to breathe air. He didn't want us to be fish.

If you want the scientific answer it is this:

The main reason is because a mammal would need to have GIGANTIC gills. Gills work for fish because fish are cold blooded. Cold blooded animals need a lot less oxygen than warm blooded animals do. Imagine that you have a 100-pound human and a 100-pound fish sitting on a couch. A human being needs maybe 15 times more oxygen than the fish does. If the human then gets off the couch and starts swimming, the human needs even more oxygen. So does the fish, but fish are super-efficient in the water.