I've always seen the Mac Pro as being a machine not targeted towards the general consumer and definitely not towards gamers. I'll attempt to explain a few points based on what I currently hold to be my knowledge on Mac Pros.
1) Price point- Industry standard software comes with hefty licensing fees, in this realm I class programs such as Photoshop CS4, Logic Studio Pro etc. These are the programs commonly used on the Mac Pro running OS X. The hardware side is no exception, Industry hardware designed for Industry hardware at Industry prices.
2)The Hardware specifically- When you look at the video cards that usually call the Mac Pro home, you can see cards like the GT 120, Nvidia Quadro4000 etc. These are not the cards found in consumer systems, they are specifically aimed towards tasks which need precise rendering and not speed rendering which is what games demand, thus the bad performance in PC games.
Secondly there is the CPU, all Mac Pro models come with Intel Xeon CPUs, these aren't found in the PCs and gaming rigs that you would buy from a computer store, because they are not aimed towards the general consumer. They are high end CPUs aimed towards servers and industry level work.
Thirdly the case design. Compare professional chassis intended for the same market demographic as the Mac Pro and a quick pattern can be seen. They all have similar case lay outs designed for cooling and easy service, slot out hardrive bays etc. Although this design can also be found in high end gaming cases the point is that the evidence shows the Mac Pro conforms with the criteria for meeting Industry and not the home consumer.
Sure macs also pack a fair amount of RAM if it is configured to, but from past experience of Mac Pro hardware specifications, the RAM tends to be geared towards reliability (ECC RAM) rather than blazing fast speeds that are required for games. Not to say the RAM is not powerful but it is hardly suited for gaming which is the point of contrast.
Operating System- Taking into account the OS and hardware integration, optimised firmware and drivers, rising from the closed ecosystem Apple likes to maintain as a vital piece of what makes the product and the performance. And then contrasting this against the intended use of gaming. Apart from the graphical issues raised above, there is the issue of comparative poor game support for mac (I personally view cider ports as illegitimate, just my opinion though) which means you'd need to boot camp a version of Windows. This negates one of the vital points of the product, the integrated software.
To wrap up, the Mac Pro is a powerful machine aimed at a different market and NOT at the consumer market which gaming rigs tend to inhabit the higher end of. Ultimately the hardware isn't suited graphically towards games and nor is the pricing. Considering windows as an imperative in the case of gaming, all important factors covered, hardware,software and pricing point to the best option being either to build or purchase a gaming rig that will work out cheaper and much better suited to gaming.
Feel free to add anything I may have left out and importantly any corrections to the claims made above.