Honestly, they do, sure they "play games" and most games have Mac versions nowadays. But if you are looking to buy a machine to play games 100% of the day then you don't want a Mac. Mac's aren't for hardcore gamers. Apple don't cater for gamers.
If you want a notebook or a desktop for playing games, you can get many times the performance for the same price you would spend on any Mac of the same cost. The highest performing GPU in a MacBook Pro is the two year old Nvidia 750M.
The current notebooks for gaming are using the NVIDIA 980M which is more than twice the speed of the 880M of last year (also not in Macs) and twice again of the 750M (in Macs). And you can get such a gaming notebook for less than $1500, spend $2000 and you'll have latest mobile i7 quad processor, 3K displays in said notebook too (see here and here).
Compare those prices to a $2600 high spec 15" Macbook pro which is still using a 750M GPU, see what sort of frame rates you get with new games like Titanfall and Crysis 3, and you see why people say Mac's are not what you buy to play games on.
That does not mean they are horrible gaming machines. So a GTX 680 is a horrible graphics card because other graphics cards can out perform it? No. It is still a pretty good card.
I agree that if you want to play Crysis at 5K on max settings, no mac or really custom build PC will work for you.
But to simply say they are horrible at gaming is just ridiculous.
NOTHING will be able to play max 5K. Not even triple SLI setups. So EVERYTHING is horrible for gaming then?