ReplaceAll in list not working as expected

This is my code.

lst = {1, 3, 5}; ReplaceAll[lst, lst[[2]] -> {7, 9, 11}]; lst 

It is supposed to get output {1, {7, 9, 11}, 5}

However, it can only get {1, 3, 5} instead.

lst = {1, 3, 5}; lst[[2]] = {7, 9, 11}; lst 

But this one above works. Can anyone explain this?