Above are six applications. They all look completely different. This is because each application was developed and designed independently by a different groups in response to competition.

MSN Messenger visually looks like AOL instant messenger
Internet explorer resembles the original Netscape designs
Media player's tries to match elements from Real.

There's no good reason for these applications to be so different.  Of course a lot of  Linux applications clone the Microsoft designs...

In the words of Tyler Durden, "Everything's a copy of a copy, of a copy."