As I understand it, you can order 400 genetically identical mice very easily. Eliminating confounds, easy access, quick lifespans (for iteration) and mammals with at least some transferability to humans is the primary reason. Plus, unlike monkeys, no one cares.
Some groups care, and I certainly don't want the mice treated cruelly. But I also am willing to let some mice get experimented on to save human lives. I feel most people are.
I've seen protests against and heard about attempts to free lab monkeys. To the best of my knowledge, there are no similar issues around mice in any significant numbers.
As a pet rat owner of over 8 years, medical research on rodents actually rarely results in benefits for pets of that species.
The main advantage we have found so far is that drug studies done on rats can serve as a reference for safe and potentially effective dosages, as there's very little veterinary literature on rodent care otherwise. All meds for rats are typically repurposed cat, dog or human meds and determining the dose needed for treatment is trial and error more often than not – even in the hands of an experienced vet.
Came here to say the exact same thing, I'd love for my rats to live even 6 years, I always find it a little disappointing so much research is done on them yet they still live such a short amount of time. I suppose a heart that beats at 320bpm doesn't help...
Also: much love to my fellow ratto parents out there, best pet on earth imo.
I don't know how enviable is that on average. Most experiments don't end up with a discovery, let alone a discovery of something beneficial.