Why is there still doubt that the supernatural is real?

I don’t get why people still argue if there is something supernatural out there or not. But isn’t it easy? I mean, we live on a floating rock, why can’t a mermaid live in our oceans? We haven’t even explored all of it and still people act like it’s stupid to believe in the supernatural. Or to believe that aliens are out there. I mean, there are so many different planets and galaxies, you seriously think nothing is out there? Maybe it’s not a green guy with big eyes, but something is definitely out there. Unicorns exist too, I just know it!