That assumes that they want to colonize and that our definition of "habitable" matches is close to their own. Perhaps the most common form of intelligent life likes to live without oxygen. Requires very light gravity. Or has a temperature range widely different from our own.
I'd assume that if they wanted to spread out, they'd probably have the ability (like we do) to scout out a solar system before sending a ship. Maybe our 8 planets don't have anything of value for the species looking.
We can ignore all those cases. Because the fact is that carbon-based oxygen-breathing mostly-liquid-water lifeforms must exist out there since they're the only kind we're aware of. And if exotic life does not interact with carbon/oxygen/water life, then those other aliens can still expand freely. And so they should be here already.
If you look at what we know about the history of the earth's atmosphere it went through a phase of high sulfur dioxide, then carbon dioxide, then oxygen rich, then what we have today. A very large portion of that was controlled by the specific life that evolved here. It's almost a fluke that we have an oxygen rich atmosphere. There are so many adaptations that could have happened to deal with the various stages of our atmosphere.
All we know is it's possible for intelligent life to evolve under these specific circumstances. What we don't know is if these are the only circumstances or if these circumstances are particularly common.
What is carbon/oxygen/water life is the only type of life capable of developing intelligence? What if that circumstances that lead to a carbon/oxygen/water rich environment only happen once in a galaxy? And what about the other elements? What if something like iron is, in fact, super rare in the universe on a planet with water/oxygen/carbon. Could you imagine how that might impact the technological growth of an intelligent species? Could you imagine what humanity would look like if the iron age was simply impossible due to a lack of iron?
Intelligence is also a weird evolutionary fluke. Maybe there's life on other planets, but the earth went over a billion years with animals before it finally developed a species with intelligence. Whose to say that nat 20 gets rolled often or at all?
Then there's the sad possibility that interstellar civilizations are simply an impossibility. It takes too many 1000s of years to colonize anything and so no species has tried.
There are just so many factors and the likes of the Drake equation seem to just handwave it all away in a neat little broad overgeneralization.
I'd assume that if they wanted to spread out, they'd probably have the ability (like we do) to scout out a solar system before sending a ship. Maybe our 8 planets don't have anything of value for the species looking.