Mice are around because they are decended from other mammals and the dinosaurs died out probably due to rapid climate change and habitat loss brough on by a meteorite inpact around 65million years ago.
There are fossils of mice-like organisms that date back 65 million years! I’m not sure how they evolved but I guess the key to their survival all this time is that their diets are so flexible: they can eat seeds, nuts, plants, and meat.
There are lots of theories about why the dinosaurs became extinct, but the most common one is the idea that something happened that made the climate change dramatically. There is geological evidence that suggests that around about 65 million years ago, the temperature dropped sharply and the sea levels fell. This would have affected the dinosaurs food supply.
But there are other theories that disease, volcanic activity or radiation could have killed them, or perhaps it was a string of bad luck and several factors were involved! Who knows?!
Mice are not, as is commonly assumed on Earth, small white squeaking animals who spend a lot of time being experimented on. In fact, they are the protrusions into our dimension of hyper-intelligent pan-dimensional beings. These beings are in fact responsible for the creation of the Earth.
As such, they are the most intelligent life form on this planet; the whole business with the cheese and the squeaking is just a front.