Why is Earth the only planet we know of that can support human life?

A lot of sci-fi, fantasy, and even some medieval-inspired fiction features Earth as the only planet in the universe that supports life. In fact, Earth seems to be the only planet that's even capable of supporting life. I don't get it. It seems like a waste of space to just have one planet in the entire universe that supports life, especially if you have a vast, diverse universe. If Earth is the only planet that can support life, then what's the point of all the other planets? If we had a whole galaxy of planets to explore, why couldn't at least some of them support life too? It's like if all of North America was the only place on Earth that was habitable, or if only a few cities on the East Coast were the only places in the United States that were habitable. That doesn't seem like a lot of fun or a very interesting world, especially if you're planning on making a fantasy world where you can have magic and mythical creatures. It seems like there would be a lot of possibilities if there were many planets that could support life.<br><br>Also, why do people often make the conditions on other planets so extreme that not even the hardiest of humans could survive there? I understand that the conditions on other planets might be too harsh for humans to survive, but it seems like most sci-fi features planets that are so hostile to human life that humans would have a hard time even surviving for a few minutes. If you want to create a sci-fi world, then why not make at least some of the other planets capable of supporting human life? It seems like that would open up a lot more possibilities for storytelling, exploration, and even colonization. If you have a vast, diverse universe, then it seems like it would be a waste of space to just have one planet that can support human life.