There is the bandwidth issue, but keep in mind that games have highly customized routines that only do the minimum amount of work, usually only work at one resolution and in one color mode, use a specific, pre-defined memory space, clip to boundaries that are multiples of 8 or 16 pixels, and so on.
Workbench must be much more versatile. It must clip and refresh intelligently under a wide variety of conditions without corrupting the display, using too much memory, or fragmenting memory. In fact, Workbench has several window refreshing modes available to the application programmer. They all behave and perform differently.
Legacy support is also important, as many people have found out after the updates from OCS to ESC, 68000 to '020 with caches, ECS to AGA, etc. Games that bang the hardware will always perform better than intelligent code, but will also fail when the hardware platform is updated. Commodore constantly warned people not to hit the hardware, but few people listened. I really wonder how AAA would've been able to hit the market, given that it would not have been register compatible with AGA.
OS programming is not the same beast as game programming. A lot of people don't understand this, and if they did, they would never ask for custom Amiga hardware instead of standard PC hardware.
