Lol, damn, you rarely see such good questions here.

Pet food is predominantly made from the off-cuts and waste products of the industries producing animal products for human consumption. Even pet food made from premium meat depends on the economies of scale of those industries. There no industries intentionally producing mice at mass market scale ergo there is no widely available mouse meat cat food.

There was an interview with a food scientist at a pet food company where this question was asked. Their reply suggested that they had experimented with mouse-flavour cat food in their testing lab, but discovered that cats don’t actually like the taste of mice compared to the meat of larger beasts humans get for them.

People would think it was made of mice and wouldn't buy it. Even though, like most cat food, it would mostly be made out of Meat Man, the Man Who Regenerates Infinite Meat

I don't know the answer, but I've always thought it funny that we give them things like tuna and beef. Two animals that a cat would never either encounter or be able to take down in the wild. They're eating meats they'd never get to taste otherwise.

taste testers don’t want to work anymore smdh

Idk if they really like the taste of mice, as much as they like how mice jiggle and run.

Cat would chase a mouse, but eagerly slurp down a bowl or rot-stench gravy meat.

I am not a mouseologist, but i would imagine that it's due to how difficult it is to get that meat divided by how much usable meat per mouse you actually get out of the process. There's a cooking term for this involving percentages but I'm pretty high and I cannot recall it for the life of me.