Because iMovie for Mac is not free, so it isn't pre-installed on Mac. When you want to make video editing on Mac, you will take iMovie as the first choice. Expect the iMovie for Windows, it provides iMovie for iOS to edit video on iPhone, iPad and iPod.
Together with iPhoto, iDVD and GarageBand, iMovie is another component of Apple's iLife suite of software applications for video editing. iMovie claims that it can help you (including non-professionals) to easily transform videos into Hollywood-quality clips. IMovie is an all-featured video editing software created by Apple Inc.
