Because the Apple OS is not resolution independent. In the Windows OS you can change the screen DPI. Changing the DPI in Windows allows you to clearly see the text and icons with hi-res screens because all the graphics get scaled.
Apple dropped support for resolution independence from Leopard. As a result if you try and use Leopard with high resolutions on small screens you end up with very small icons and text, which leads to eye strain. Even on the Hi-res 17" MacBook Pros the OS looks too small IMHO.