First of all because they are cheap and easy. Mice can live in a small cage, don't eat much, and don't live long. Mice also can be forced to eat whatever diet you want, (unlike humans - nearly all diet studied in humans should result in "in conclusion we were unable to get the subjects to eat their assigned diet") That means you can do a study in a lab quick and cheap.

While mice don't generalize, there is a lot we can learn from them. We just need to remember the limits of mice studies and not generalize to anything other than mice (and even mice is questionable since lab mice are often genetically different from wild mice, sometimes intentionally). So long as you remember the limits mice are valuable.