Primary reason: audience. There are simply more PC users out there in the world than Mac users. If you're a company, your goal is money so you want as many potential consumers to buy your game as possible.
Another reason is customization. The Macs have much more fixed memory, processors, and video cards compared to PCs, where upgrading is pretty easy, and relatively cheap. Consequently, this also gives developers a chance to make more in-depth games wheres with Macs it'd be like programming for a console system.
It's not that developers are lazy, it's just that there's no profit in making games for Mac, so people simply haven't done it.
Despair not atomik, there is another way. Are you familiar with Steam? Steam offers tons of PC games that have also been made for Mac, check them out:
Welcome to Steam
registering and installing Steam is free, and any games you buy off Steam are saved to your account, so you can access them and install them to any computer you want. (as long as the minimum specs are there, of course.)
You could also suck it up and get a Windows OS for your Mac.