I know VERY little about Ego and Galactus. 
So why does Galactus allow Ego to live?

It might be because he was an Elder of the Universe (I don't know if the Elders are still "elders," since Thanos took their Infinity Gems). 
It does seem like Ego would be the ultimate snack for Galactus though.  Ego is mobile too, so maybe he's constantly on the run from Galactus, and nobody has mentioned it yet.

@cbishop: Ego was never qualified as a member of the Elders of the Universe, as he was not the remaining member of a deceased ancient race. In particular, the Elders were of the first populations in the galaxy, behind the Abstracts, Galactus, the Celestials and Primordial Gods. Ego was just a planet that the Super-Ego bonded. Now it has retconned that the Stranger had created Ego and sibling planetoid as experiments. Marvel gets really lazy with the retconning.

@lars_maximus: I'm honestly not sure either way, but what I was recalling at the time was when Thanos was first after the Infinity Gems, the Elders met on Ego to strategize, because (I thought) Ego was one of the Elders as well. I'm either wrong (quite possible), or it's something that got retconned out.

@cbishop: The Elders initially were introduced in Silver Surfer series as nemeses who each had prolonged their lifespans via intense dedication to a single passion, which became their nomenclature like the botany of The Gardener or the games of The Grandmaster. That n their sole existence as the last member of respective extinct species is why they are 'elders'.

The Grandmaster duelled Death in board game and lost, banning the Elders all from her embrace and dimension forever, so they all became immortal in that way, never able to pass through into Mistress Death's domain. They can be disintegrated into millions of particles but still maintain a loose consciousness, such as when the space dragon humanoid Reptyl dismantled the Contemplator. The Grandmaster, with the power to reanimate dead souls, time travel and manipulate reality, often has the role of reforming his 'brethren'. The roster of Elders includes Grandmaster, Champion, Contemplator, Gardener, Collector, Runner, Astronomer, Trader, Caregiver, Trader, Judicator, Obliterator, and Possessor and as they cannot die, it is assumed they are all active in the Marvel Universe. The comicvine page lists Ego and the daughter of the Collector but I already explained how they don't fit qualifications.

By the way, why Galactus doesn't eat Ego is due to plot device. Whenever Galactus is weak enough to benefit from the life energies of Ego and sought it out, the World Devourer encountered Ego's considerable psychic/defensive powers. However, a powered Galactus has the power to take Ego down but isn't in need of those energies. And as an Abstract, Galactus wouldn't take on an opponent/energy source without necessity. Recall how consuming the Phoenix Force would sate Galactus unending hunger but at the cost of the recycling of the universe and stars that is unavoidable.

http://i238.photobucket.com/albums/ff240/Branlactus/Raw%20power/Tech/Excalibur_025_-_20.jpg

@lars_maximus: Alright, I had to do a little reading here, but Ego is actually a known member of the Elders of the Universe, from in-story elements. Although he doesn't possess an Infinity Gem, he qualifies in that he's the only one of his kind. I agree that it's sort of a lazy retcon, but if the actual comics say he's an Elder, he's an Elder.

@cbishop If you could scan or post the image where it says as such I would love to see that. I hate when writers contradict other writers with complete disrespect, like when Red Hulk puched out The Collector or the many MANY times that people have survived direct attack by Darkseid's Omega Beams even they are supposed to be OP.

@lars_maximus: Check out the wiki entry - that's the reading I did. The scene I was remembering was from the original Thanos Quest.