Obviously you don't grasp what genetic modification does to mice and rats. I'm sure you have a frankensteinish vision of it (like many people do), which is far from the truth.

Genetic modification usually targets only a specific gene to study one specific metabolic pathway. Some modifications can improve the animal's health, and some don't. The EXACT same thing happens with humans naturally or artificially (pollution, exposure, genetic diseases, etc), and understanding the cellular mechanism helps in the development of cures. I hope you find this enlightening.