Because `do` binds the farthest away. In these examples:

    case some_fun(1) do
    end

    case some_fun() do
    end

    case some_fun do
    end

We all know `do` binds to case. Therefore, if `fn` used `do`, the `do` would bind to the farthest call to:

    # this code
    Enum.map list, fn do
    end

    # or this code
    map list, fn do
    end

would both bind `do` to `map`.

Of course we could special case `fn do` but having `do` bind to different places based on `fn` would be extremely confusing.