Because very few people game on laptops compared to desktops. Plus really the laptop is limited on space and power delivery so most of them do not have the resources to run good power hungry GPU's. Most laptop buyers are interested as well in battery life which is very short with a good GPU.
Some people don't game, and thus have little use for a good GPU.
Some people do game, but have no idea that the GPU is crucial for (3D) games. So they go for a good CPU.
Both those groups probably make up a larger share of the Mac users.
Because very few people game on laptops compared to desktops. Plus really the laptop is limited on space and power delivery so most of them do not have the resources to run good power hungry GPU's. Most laptop buyers are interested as well in battery life which is very short with a good GPU.
I think good GPUs are hard to come by performance wise but also accommodating to design. However, you will find specific lines from gaming laptops that would weigh like 9lbs versus the traditional 5lbs with much more loaded laptops.