There are two issues that are not generally considered, the first is that it may be that the normal size of intelligent beings is rather larger than humans. Sec

There are two issues that are not generally considered, the first is that it may be that the normal size of intelligent beings is rather larger than humans. Secondly it might be that the size of the worlds that the live on is larger than the earth. Bigger worlds will have bigger gravity and might have thicker atmospheres, possibly less deep though. All of these factors would make space travel much harder, life support would be more difficult. take off harder (much) return to the surface much, much harder. So I think that even if civilisation is common space faring civilisation is going to be rare, and as we know even space faring civilisation doesn't approach interstellar civilisation.