Most of the selective breeding has been done while these animals were on simple diets

I am not so sure of that. People have been feeding cats and dogs commercial pet food only for the last 50 years or so and only in wealthy countries. Before that (and in the rest of the world, still) people fed their pets a variety of food that doesn't come from a bag or a can.

aren't carnivore diets quite monotonous in nature anyway?

In terms of what you kill and eat, mostly yes, but in terms of (micro)nutrients prey not only differs, but also each body contains a huge variety (compared to plants).