As Paul says, the sentence is saying that it is fun to find the stars. If you said it was fun "to be found," this would mean that the stars themselves are enjoying being found. Of course, the stars are not the ones who are having fun being found. People are having fun finding the stars.
The same structure works with other verbs. A song can be "fun to sing," a car can be "easy to drive," a math problem can be "hard to solve." It doesn't make sense to use passive voice in any of these cases.