To answer your question:
If we can have inner private class then why can't we have outer
private class...?
You can, the distinction is that the inner class is at the "class" access level, whereas the "outer" class is at the "package" access level. From the Oracle Tutorials:
If a class has no modifier (the default, also known as package-private), it is visible only within its own package (packages are named groups of related classes — you will learn about them in a later lesson.)
Thus, package-private (declaring no modifier) is the effect you would expect from declaring an "outer" class private, the syntax is just different.
