That is a good question. It is true that there are thousands of confirmed exoplanets, but the arrangement is different from the solar system. I would like to begin my response with a humorous look at the beginning of the mediocrity hypothesis.
Kant believed that God did not intervene in nature and particularly at the scale of the solar system. His argument for the nonintervention of God was the mediocrity principle, which he based on his belief that all of the other planets in the solar system were inhabited by humans. He reasoned that if all of the planets had different angles of obliquity (different axial tilts), then God did not set the angles of obliquity of the planets at optimal angles for humans. If God had intervened, then Kant reasoned that all of the planets would have the same optimal angle of obliquity for humans… And thus we have the beginning of philosophical naturalism.
Kant aside, the exoplanet data is suprising. Most other planetary systems have hot Jupiters or super-earths orbiting near their star. Read the comments of leading planetary scientist, Kevin Walsh:
“We have no idea why our solar system doesn’t look like these others, and we would love an answer,” said planetary scientist Kevin Walsh, of the Southwest Research Institute in Colorado. Since the time of Copernicus, scientists have slowly moved Earth out of its originally-conceived setting as the center of the Universe. Today, scientists recognize that the Sun is an average star—not too hot, not too cold, not too bright, not too dim—situated at a random spot in a typical spiral galaxy. So, when Kepler began its planet-hunting mission in 2009, scientists anticipated finding planetary systems that resembled our solar system. Instead, Kepler mostly discovered planet types that our solar system lacks. With bodies like “hot Jupiters” (Jupiter-sized planets that orbit their star in only a few days) to “super-Earths” (massive rocky planets far larger than our own), exoplanet systems have a knack for surprising observers. Of the 1,019 confirmed planets and 4,178 planetary candidates identified to date, only one system resembles our own with terrestrial planets near the star and giant planets set at a distance.”
On the theoretical side, earth should not be dry. Leading planetary scientists, R. Machida and Y. Abe, described the problem of dry earth formation:
“Models of terrestrial planet formation have been based on the assumption that the formation of planetismals occurs in a transparent (optically thin) nebula, in which H2O ice is unstable at the formation region of the terrestrial planet due to direct stellar irradiation. However, in the astronomical context, it is confirmed by both observations and numerical models that protoplanetary disks are initially opaque (i.e., optically thick) owing to floating small dust particles, and the interior of the disk is colder than the transparent disk. If planetismals are formed in opaque cold nebula, they should be mainly composed of H2O ice, even at the formation region of terrestrial planets.”
Recently, leading planetary scientists Batygin, Laughlin, and Morbidelli made the following statement about the architecture of the solar system in comparison to exoplanets in the local part of the galaxy.
“The solar system’s configuration of small inner rocky worlds and large outer giants is anomalous in comparison with most other planetary systems, which have different architectures.”
Based on the fact that our solar system is anomalous among other observed planetary systems, an increasing number of scientists are beginning to believe that we are essentially alone in the universe. For example, Howard Smith, senior astrophysicist at Harvard made the following statement.
“We are alone in the universe, at least for all practical purposes. This is the most probable conclusion to be drawn from a host of fundamental physical constraints and new astrophysical observations, and in particular the discovery (at this writing) of 4,696 exoplanet candidates (http://kepler.nasa.gov/) including some Earth-sized planets in their habitable zones (Quintana et al. 2014). The implications of these discoveries, and their modern context, are radical.”
When Smith stated that we are “for all practical purposes” alone in the universe, he did not mean that there are no other civilizations in the entire universe. Astronomers have only confirmed several thousand exoplanets in our local part of the Milky Way, which only represents a tiny fraction of the planets out there. Even if our earth is the only planet with life in our whole galaxy, there are billions of other galaxies in the universe. It would be unfair to those unknown civilizations to dismiss them as non-existant only on the grounds that we have not yet observed them; as far as we know, they may have already dismissed the possibility of our existence based on the lack of contact, and yet here we are. “For all practical purposes” means that civilizations are either nonexistent or so rare that there is almost no possibility of ever coming in contact with them.
Brad Hansen from UCLA showed in 2009 that if there is a gap in the circumsolar disk between earth and the asteroid belt, then the inner planet architecture forms as it is (small Mars).