Even people applying for U.S Citizenship have exemptions like if your over age 50 you don't need to pass an english test and you can take your history test in your native language. Also remember there are many classes of people who reside here besides permanent residents. There are many nonimmigrant classes where theoretically they are here for temporary periods and must return home but many stay for very long periods of time and eventually become permanent anyway. Also there are many who derive and acquire U.S Citizenship and there are no tests of any kind for them.