How do you know the solar system represents every possible combination of habitat? How do you know life can only exist in the forms and manner you are aware of?
I don't
know. Do you
know that there are other totally different habitats that support complex sentient life?
But we do have
most possible habitats them right here in our solar system- most are too hot or cold or unstable, no atmosphere, no rotating core, magnetic shielding, no stabilizing satellite, not enough water, stable crust, just to scratch the surface, science continually learns how amazingly special and specifically designed for life Earth is.
Aliens seems intuitive I know, in days of old we traveled to exotic locations and found exotic people, and in those days of Verne and Poe, we debated what sort of folks inhabited the moon... because we found people everywhere else
But our knowledge has come a long way since then, there are very good reasons other planets are devoid of even the simplest life, it's extremely fickle relative to the excruciatingly narrow set of required parameters