This is intentional for privacy reasons. If you would be able to get the (real) User ID with the unique username, there would be no need to invent App Scoped ID at all.

如果您做出调用 / me / username ，它将返回以下错误：

If you make a call to /me/username, it will return the following error:

＃803）无法通过用户名查询用户名

(#803) Cannot query users by their username

Btw，你甚至不能从v2.0开始获取用户名，只有真正的名字。

Btw, you cannot even get the username since v2.0, only the real name.