It is not always so… Nowadays even Microsoft offers software that has the “image” of Microsoft’s Office, but is fully acceptable by Macintosh as well. I am talking about
Microsoft Office v. X for Mac

It offers full compatibility and integration with Mac operating system. Many Macintosh users claim that with present technical possibilities it is easier than ever to exchange information between Macintosh computers and Windows based computers.
“Office 98, Office 2001, and Office v. X for Mac — as well as Office 97, Office 2000, and Office XP for Windows — all use a consistent file format. This means that a file created by using any of these versions of Office will open in any other version with no special conversion. If you are using a version of Office for the Mac, you can even automatically add the correct file extension when you save the file so that it can be opened by using a Windows version of Office.”1
Each of the Office v. X programs has features of a new Aqua user interface that's completely in step with Mac OS X. It does not touch the basic part of the software – means Office package as Word, Excel etc. They provide a connection with developing Entourage X – through it one can efficiently send email messages and it provides connections directly to Microsoft, Hotmail and Mac web sites that one can efficiently use.
Speculations about Macintosh that were recently released:

According to the weekly magazine PROFIT No. 41 – The founder and the director of Macintosh organization concede that the future version of Macintosh will be able to run on the processors from Intel as well – inside of Macintosh will run Pentiums. Analytic Andrew Neff supposes that the change will take place within 2 to 4 years. He gives 80% possibility of its happening. Some of the analytics are critical about this change. Macintosh is ahead of Microsoft mainly because of its PowerPC processors, that is made by Motorola. But Motorola is, as it seems to be, the main problem. Motorola is mainly developing processors for mobile phones and thus is sometimes forgetting computers. The redactor of weekly magazine “The deal” Andrew Sasaki said that it is possible to use alternatives from both suppliers. There are of course other opinions about it. The redactor of weekly magazine “Business Week” Charles Haddad thinks that the Mac would lose its unique. At the present moment they are fully controlling the software as well as the hardware parts of computer. While Microsoft has many producers for its components and thus one program may perfectly works on one computer and breaks down on the other.