It's simple. We're the first intelligent life in the galaxy. There is nobody else yet in our galaxy.
Also, probably nobody capable of traveling the stars wants to settle a planet. Once you figure out how to make huge spaceships (which you'll need to travel interstellar space) you've essentially learned how to make cities in space. Our solar system would support a lot of people if we just used the resources available for space habitats, and by "lot" I mean in the quadrillions. And it turns out that all you need to support that population is a star to provide energy, and some planets to source materials from.
So with that in mind, why bother finding another habitable planet?