This is great question over which i was wondering also. At first let me say that most anwers here fails to recognize the real problem pointed out here in part because the answer is clumsily formulated. The better formulation which comes to mi mind is:
Why it is that planets which support some life and we get to see them in star wars movies and other media are of such nature that almost all are habitable by human. In other words, how it is that most of aliens we get to see demand same enviroment as human
At first note that rare exception like Plo-koon does not count to counterpoint the question, as we dont ask for all planets and aliens but most. Even Plo-koon share same need of presure, heat, gravity and other things. He just cant breathe but dont have problems with other parameters.
I came to some answers, which are itself dubious but make some sense:
1) Humans may be one of the oldest inteligent specie in galaxy and so they were able to colonize more planets than any other specie. (But that does not mean more than other species combined.)
2) Humans may belong to most probable inteligent life-form to evolve, in other words, earth like conditions may be most probable enviroment to support life and many of the lifeforms could evolve from the simmilar planets. (Yet this does not mean most of all, although earth like conditions could be the most probable condition it does not mean it would be more than the others combined) (Alsp there are species like space-whales, so pretty unprobable one)
3) It may be that most of habitable planet are product of teraforming from some even older species like Rakatan empire or Sentinels.
4) It is possible that humans or other specie which need similar enviroment just force most of other planets to teraform and than force native to adapt by genetic enginering. 350
Although there is some tendency to human supremacy, it seems unprobable. And if the humans were so abusive to change enviroment of strangers planets, why would they care to modify them and dont let them die? And how would they change gravity?
5) Humans or some older specie with need of same enviroment just destroied most of planets with lifeforms incompatible enviroment.
6) It is the will of force to unite all species together
7) It is humans who are modified to survive most conditions. That is unprobable because we see most of other species to interact together on Coruscant.
8) There is big number of planetary life with enviroment non-compatible tu human yet we dont get to see those planets because we follow human heroes. Well it seems possible but if there is big number of such planets, than they should have some influence in galactic politics and warfare so our heroes would have to go there. Also we dont see much of the species with technical life support in senate.
9) It may be that those species are ignored by galactic goverment. But how it can be, that they are so weak that they do not force the goverment to count with them. If we say there is more of them, than the rulling one, it is unprobable.
10) All of the above combined.
So those are my ideas. Get me know if you have some others or if you have some counter arguments against my ideas.
Also please forgive my english and spelling erors.
Thanks and have a good luck 350