Essentially, the initial state of war between factions in the game is a case of contextualization. There would be no game situation if things hadn’t broken down to a state of mutual hostility between all at the table. That’s the context; talks have failed and war is onrushing. Thus, you’re given one planet, minimal resources for the area and told “Go save the bacon, young man/woman/reptile-thing!”
Frankly, you might as well ask why you only start the game with one planet and not a vast panopoly of resource-rich worlds ripe to squeeze to fuel your hideous war machine. Because that’s not how it goes in the SP game and definitely not in the MP game.
Changing initial situational context is the place of scenarios, custom maps with different setups, etc. Which can and will be made (likely moreso once we get some better documentation on the scrupting system, etc). Within custom galaxies/maps, you’ll see a lot of variance.
If you want to play with friendly AIs right now, there is an easy way: start a SPgame. Set unlocked teams, then change a couple of the AI’s to be on your team. With unlocked teams, they can fall off but you start off with a Peace Treaty and it’s all downhill from there.
Good luck, soldier.