Given the absolutely enormous number of planets in the Universe (there are about 10
11 galaxies each with 10
11 stars, and who knows how many planets per star?), the chances are
extremely likely that some planets will be in the habitable zone by pure luck. We've already discovered
quite a few Earth-sized planets in the habitable zone of their star, and estimates suggest there may be
8.8 billion terrestrial planets in the habitable zone of their star in our galaxy alone! Given that some planets will end up in the habitable zone by chance, it's not surprising we find ourselves on one of these - since we as living beings need to be on a habitable planet by definition.
It's an effect called
anthropic selection - if we find something has a low probability (but not so low as to be almost impossible) and had to be necessary for a planet to be habitable (e.g. being in a stable orbit in the habitable zone of a star), we shouldn't be surprised to see that that unlikely thing applies to Earth - its small-but-not-too-small probability combined with the enormous number of planets in the Universe means it had to happen on a few planets by chance, and Earth had to be one of those planets or else we wouldn't be here to talk about it!
Personally I've always been baffled when people use the Earth's orbit as an argument that Earth was specially designed as a home for life. If Earth was the only planet in the Universe, then Earth's habitability would be a rather spooky coincidence. (Maybe. Or perhaps that would no more prove the Universe was designed for life as it would prove the Universe was designed especially for plate tectonics, or continental crust, or surface liquid oceans, or a nitrogen-rich atmosphere or other features of Earth - perhaps life would just be something that happened to arise on Earth with no particular cosmic significance to it.) The argument falls completely flat, however, when we look at all the other planets in our solar system and the vast majority of the exoplanets out there (not to mention the huge empty spaces between solar systems with no stars or planets at all, or the regions of the galaxy that are too low in metals or too high in radiation to support life even if Earth-like planets exist, or...) and realize that they
can't support life. Why should Earth be so privileged? Why assume Earth was designed for life, when chance alone seems to be perfectly capable of explaining its orbit and the vast, vast, vast majority of the Universe clearly
wasn't designed as a home for life? It would be as if the Creator built a city full of structurally unstable buildings, only one of which had enough strength to stay intact after a tornado rips through the city, and pointing to that one building as evidence that the whole thing was created for the purpose of withstanding tornadoes.
(None of this says God doesn't exist, by the way, only that you can't use the "design" of Earth's orbit as evidence for that. Science seeks to explain the world in terms of natural phenomena, and makes no comment on the existence of God - science and religion are two different things, designed to answer different questions. Earth's orbit and habitability can be fully explained by science, so can't be used to bolster a design argument for God's existence.)
One final thing: the habitable zone isn't necessarily the only thing making a planet habitable. You have to consider the planet's atmosphere, its size, its water content, abundance of organic matter, long-term climate stability... then there's the evolution of those planets over time (Earth's remained a pleasant home for life for at least 3.8 billion years, albeit with a few mass extinctions, but most planetary scientists agree Venus and Mars were once habitable too and are no longer) and the fact that with tidal heating some planets or moons outside the conventional habitable zone may still be capable of supporting life (Europa, Enceladus, Titan)... not to mention alternative and hypothetical biochemistries!
