It's probably just their natural fur coat. These are genetically engineered/modified monstrosities, after all.
Clothes aren't genetic, unless you're modifying the textiles before stitching all them suits together. *Alien Suit Stitching Division*
It might just be a kind of uniform, seeing as they're all identically equipped with the same outfit. Alternatively it's so that they look more like civilians, which would give them some more tactical options that wouldn't appear in the game.
Perhaps the Aliens could mistake business attire for having more meaning than, "
I sit in an office and type all day," but its doubtful. Making themselves appear to be human civilians offers zero tactical options on a battlefield while fighting against XCOM operatives and presumably normal soldiers, which they know they're going to end up doing when assaulting cities. Maybe early on we might have mistaken them for humans and been punished for it in a token engagement or two, but midway into the battle for Earth's survival, when we've been killing these losers for months? Yeah, right. And its not like they're trying to fake it in the first place either, what with diving around and jumping and crap whenever you see them. Another thing, if they all look exactly the same (which is to say,
they do), how hard would it honestly be to simply hunt their asses down when they try to infiltrate our cities? If they were only infiltrating our cities before and during the beginning of the invasion, having realised that their disguises no longer work, why do they still wear the suits into normal engagements? Do the Aliens have a thing for Men's Wearhouse or something?
Also. Remember those green metal pods from the intro? What do they actually do? It was fairly ambiguous IMO.