I finished a graphics course a while ago, and I have been working with graphic designers for over 4 years. Every single one of them needed a PC because on Mac, something was missing. Guys who were bringing Macs with them needed a PC to finish some projects. Guys working professionally with graphics (TV commercials, 3D games, animation, and other stuff like that) were all using PCs because Macs couldn't cover all their needs. This is just my experience, but it highly depends on the project. If someone is doing only one specific task on a single program then it doesn't matter. Worse is when there are multiple programs involved.
Most programs can be replaced, but most users have their specific environment and don't want to learn everything from the beginning.
Another thing is the price. Macs and specific software cost much more than a similar environment for a PC. If a graphic studio has multiple computers then it's already a problem (unless has a huge budget).
This is my experience and my point of view that doesn't have to be the same for every other person.