Firstly, because it controls a little differently. But, more importantly, because of the player's character, and the game's setting.
You're not a space marine. Or a US Marine. Or a cyborg, bounty hunter or cowboy. You're...a balding old British geezer, a crackpot alien invasion nut who has the last laugh when aliens invade and start eating everybody's face off.
Note also the alien's resemblance to a certain other Ubisoft franchise, one synonymous with Nintendo consoles...