To put it back on topic 7-zip is a free open source program that easily opens rar files as well as many others.
7-zip is ok...WinRar though is far superior, imo..it does infact cover 7Z, ACE, ARJ, BZ2, CAB, GZ, ISO, JAR, LZH, TAR, UUE, Z files..
The only real advantage to the "Zip" format is popularity...
Rar on the other hand has much better compression than Zip, it supports multi volume archives, Waaaay better than spanning a zip file...
You can rebuild damaged data, and you also have the ability to lock your Rars so they dont get modified..
Also, RAR breaks the 2gig Zip archive limitation, you can RAR files over 8 million gigs if thats your "gig"..LoL
So to answer your question...Thats why,,,LOL