>>59273
hostile factions teaming up happens in 40k too, space marines team up with tau to kill tyranids, necrons team up with mechanicus, eldar team up with imperial navy. there's literally a subfaction of orks that will work with other species for gold
>why can’t they put aside their differences for both their sakes?
because every factions goals are so different/hostile to each others goals that teaming up puts both sides at a disadvantage in the long run.
tau can't expand because a tyranid hive fleet ate all the worlds around them, so are forced to expand into imperial space for territory and resources.
elder are too weak to be good allies, deldar don't want to risk being eaten by slaanesh by making allies, harlequins are retarded.
necrons hate other lifeforms and want to dominate the galaxy by themselves, they would never ally with another race (excluding trayzyn)
imperium and mechanicus are bound by the imperial truth to enact total xeno death, so only work with aliens if they aren't religious zealots.
chaos is chaos and tyranids are tyranids, but some tyranid fleets will ignore non chaos factions in favour of chaos factions, because they're specially adapted to fighting chaos.