Why did Microsoft make office for mac if it's already so compatible with the PC version?

You have to buy software for the PC just as you do with a MAC. I assume you are referring to Microsoft office? If so most PCs do not come with a full version and it must be purchased separately just as on a MAC.

It is true that Apple's computers are very compatible. For example Pages can open and export to Microsoft Word doc files.

Microsoft sells Microsoft Word to Mac owners in order to make money. Microsoft believes that some people want 'the real thing.'

The iWork applications, Pages, Numbers, and Keynote, cost Mac users USD$80.

The Microsoft Office 2007 Home and Student CD retail costs USD$108