There's no technical reason why they couldn't - though I guess microsoft would have something to say about it. I was under the impression that there is a company that provides a compatibility layer for game porting that firms can licence but the name escapes me right now. The graphics API that the Mac supports is OpenGL - OpenGL is also supported on windows (it's used by the Quake series for example) but MS is really pushing DirectX as it ties developers into the windows platform. As to which is better...well I've always thought OpenGL was a really easy API to work with and DirectX in it's early versions was hideous but it's got a lot better and seemed quite reasonable last time I looked.
Apple doesn't support any MS Direct anything; however, game developers usually have their own libraries to cover the important pieces to allow porting of titles. OpenGL is usually better and easier than Direct3D but Microsoft can quickly react to new cards and technologies, in contrast to the OpenGL ARB which moves with the speed of bureacracy to get things just right. Apple still supports portions of DrawSprocket and NetSprocket, which were part of their response to DirectX.
Currently DX9, and soon 9.1, have an edge in features over OGL (currently at 1.4). Sadly, both ATI and NV use DX as a blueprint for what features to add to their cards. In cases where chip makers ignore MS's guidance (like the ill-fated NVidia GF FX series) the results can be disastrous. Those nice video cards in our systems are practically designed by MS now (as far as features are concerned).