Animals who see infrared are cold blooded. My guess is that the hardware that evolution can easily produce to detect and process infrared gets flooded by the heat of warmblooded animal and is therefore not effective for them.
We could in theory avoid the problem if our eyes were at the top of antennas.
But yeah, genetic evolution does not explore states widely different from its origin. The more complex an organism, the more "conservative" the entire process is, and we are quite complex.
