Why await is not allowed in a finally block?

Why await is not allowed in a finally block?

public async void Fn()
{
    try
    {
    }
    finally
    {
        await Task.Delay(4000);
    }
}

knowing that it is possible to get the Awaiter manually

public void Fn()
{
    try
    {
    }
    finally
    {
        var awaiter = Task.Delay(4000).GetAwaiter();
     }
}