Why cannot we install microsoft excel/word from Mac OS X, since it is a BSD/UNIX?

I mean BSD and UNIX, arent that far from LINUX

It isn't enough that the OSes are in the same family. Office for Mac depends on a lot more than just being able to manipualate Unix-style permissions; at a minimum, it uses the OS X graphics platform (Cocoa, I believe), which is not available for Linux. I'm sure it also needs many other libraries and interfaces which Linux won't have.

By the way, you should read up on the relationships between Linux and BSD and between each of those and Unix. Doing so might make this more obvious.

Not to mention the executable formats are different: ELF with Linux and most Unix/Unix like OSes but Mach-O with Darwin (Mac OS/X).

If I understand it correctly, Mac OS-X is built on top of BSD. I think you can run programs designed for BSD, but not programs designed for OS-X

You can recompile most of them ( http://www.macports.org/ http://www.finkproject.org/ ), but you can't run them directly.
As I already wrote, the executable formats are incompatible between Mac OS/X and the rest of the *nix world so there is no supported way to run a BSD, Linux or whatever binary under Mac OS/X outside using some kind of emulation/virtualization like VirtualBox or experimental hack like xbinary ( http://osxbook.com/software/xbinary/ ), and there is no more way to run a MAC OS/X binary on a BSD, Gnu/Linux or Unix based OS as there is no support for Mach-O there.

There are dead or unfinished projects though:
http://hcpnet.free.fr/applebsd.html
http://lkml.indiana.edu/hypermail/li...03.3/1852.html

As karamarisan already stated, Excel and Word are using Apple's proprietary graphical and other APIs so frenchn00b idea is mostly doomed.

By the way, Mac OS/X isn't strictly built on top of BSD. It incorporates some of BSD kernel code as a service running on top of a micro-kernel (MACH) and provides userland BSD utilities.