The spirits of Aang and his past lives are reborn through Korra's body..

<p>Maybe none of them wanted to? I thought the "spirits" the avatar could communicate with were just imprints of the former avatars, instead of their actual spirits, sitting around silently waiting for the next avatar to need them for a minute or two of conversation before going back into an "and I must scream" scenerio. </p>

<p>The previous avatar spirits seem to exist in their own plane, accessible to the avatar through Rava. But did we not see Tenzin see the spirit of Aang in the fog of lost souls? Also who is to say other spiritual people were not in the spirit world? It's a virtually limitless plane of existence, their could be thousands, millions of spirits we never see. Iroh made himself apparent to Korra for story plot and rememberance, but other spirits may choose to ignore humans. </p>

<p>Aang could not attain immortality in the Spirit World because that would prevent him from reincarnating into Korra. No, the Spirit World is not "heaven," becoming a spirit is an option that particularly spiritual individuals have, not a final destination. </p><p>It's implied that the Aang which appears to Tenzin is a vision, not Aang per se. </p>

<p>Humans who choose to live in the Spirit World become more and more "spirity" and less like their physical bodies, Just look at the Painted Lady, and in a few hundred years they could be nearly unrecognisable from non-human spirits. There could also be lots of human spirits the protagonists haven't encountered since the Spirit World seems to be a huge place. </p>