It's an instinctive behavior. The main reason cats catch mice is that they need to obtain taurine from mouse meat, a substance that helps them with night vision. Besides mice, cats also catch birds and fish, but they catch mice more often, mainly because mice are the easiest thing for cats to come into contact with. This is also related to cats' habits; both cats and mice are nocturnal, and mice are plentiful and reproduce quickly, making them very easy for cats to encounter.
Generally speaking, cats catching mice is more common in rural areas, since the countryside is where mice frequently appear. In cities, however, cats mostly live indoors and have fewer opportunities to come into contact with mice.
