Being an absolute geek in everything aliens and predator, i can answer this.
In order to adapt to their living environment as inhospitable as it can be, the alien quickly match their body heat to the ambient temperature, making it impossible to detect heat from them. Besides the xenomorph's body composition is a whole lot different from animals on earth espacially when they have acid for blood and the ability to live with molten lead pouring on them in alien 3.
Also the concept of the predator's vision mode was base on the original avp comic prey. (unlike the movie its actually rather good), the predator have to switch vision mode to see different creatures. to put case in point, one of the predator that was stalking human on thermal got his ass handed to a pack of aliens because he can't see them. As such later concept of the predator vision mode as seen in movies and game was based on that.