C＃线程睡眠和并行

Question

关于以下代码示例，为什么Thread.Sleep使代码按顺序运行，但是如果我删除该行并行运行？

private static void Main(string[] args) => UsageParallel();
public static void UsageParallel()      => MainParallel().Wait();
public static async Task MainParallel() => await Task.WhenAll(Named(), Named());

public static async Task Named()
{
    Console.WriteLine("named start");
    await Task.Delay(2000);
    Thread.Sleep(2000);
    Console.WriteLine("named end");
}