There is only so much you can do on Earth. Gravity remains the same. Weather is largely predictable. We know what human beings are like. Maybe we’ve gotten tired of all of that. Maybe we feel like we know everything that Earth has to offer and we’re ready for a taste of something else. We want to see what it is like in a place that isn’t here. We want to know what it’s like in a different world, with different types of creatures, and with different rules of civilization. 

We know what humans are like, and what it takes to make us happy (maybe). We’d like to know how it works for aliens. What they’re into, what kinds of languages they speak, and what music looks like for them. This is why we reach for science fiction. This is why we want stuff that is a little different from what we’re used to. It’s all about exploration.