If you look into the lore, there certainly were other advanced races. Unfortunately, the Hive exterminated many of them over the past several millennia. It seems wherever the Traveler goes, Darkness and destruction inevitably follow because the Hive want to destroy the Traveler. It's part of the pact they made with the Worms. In Destiny 1, we fought the Hive because they straight up wanted to kill us all for the reason above. They will never ally with us, and the only way the fighting will end is with their complete and utter extinction. If the Hive do not kill and conquer, their worms will eat them. So their very existence is now dependent upon killing and conquest. We never understood why we were fighting the Cabal. They just showed up and made a base on Mars, but never made an overt offenses against us until D2. There were hints that the Cabal were running from something, but it would seem that thread was scrapped in favor of the story they gave us for D2. It's still possible for us to work with the Cabal. There are ways, Im sure, that Bungie could write a story where our species work together against a greater foe. The Vex... well, the Vex have no need to launch offensives against us: they simply wish to ensure their perpetual existence and only engage us when we make effort to obstruct them. They're not likely to ally with us at all except perhaps to use us to save themselves, as when they used Pradyth to lure us into the Vault of Glass in order to destroy the Taken presence there. Lastly, the Fallen. Here is a race we actually have the best chance of allying with. In centuries past, the Traveler actually uplifted the Fallen (then called the Eliksni) much as it did Humanity during the Golden Age. And, just as happened to Humanity, the Hive eventually showed up and caused the end of the Eliksni golden age. The difference is the Traveler fled the Eliksni, leaving their homeworld to be destroyed by the Hive and the Fallen themselves to become a spacebound race of pirates and scavengers. They tracked the Traveler to our solar system and want nothing more than to take it back to restore their people. Noble intentions, if with questionable methods. We know that the Fallen have all been united under one house now, the House of Dusk. There is no more infighting between the Fallen. Now, the world quest on Titan (Enemy Of My Enemy) has a couple of different endings, depending on who you kill first during the final fight. If you kill the Hive knight and leave the Fallen captain alone, he actually does not attack you. Instead, he postures and growls a bit, then leaves, giving you the reactor you were after. This captain wears the colors purple and gold, much like the old Queen's Guard. It's hinted heavily that the Queen's brother, Prince Uldren, is actually now in charge of the Fallen, and the fact that this Fallen boss is wearing the Queen's colors supports this. This is our best evidence at a possible alliance with the Fallen, and really the Fallen have the most to gain as well. But ultimately it's up to Bungie to decide whether or not to pursue that possibility.

Isn’t it odd that they built a wall to keep out aliens that use air ships, ships that can even go to space? I didn’t read the lore, so maybe that’s explained somewheres.

The awoken?

They were until Calus arrived. With the Red Legion broken we still have the rest of the Empire to fight, but if we put Calus back in power we'll probably get the Cabal as allies. The Fallen already have hints about a future alliance. The Vex... I'm not sure... The Hive won't ally with us, period.