Because the airflow is through the evaporator core before it gets to the air inlet door I think. Monitoring the evaporator with A/C off, it's always a couple of ° minimum above ambient in summer, and a lot above ambient in winter.
I can't bring myself to use auto; it never puts the right temperature air where I want it. Cold days I need heat down at my feet, then up at face to warm my hands for a bit, then usually temperature dialled down with low fan speed on face to keep me awake (I'm old
). I just hit recirc. when I'm following a diesel, or someone ahead on a dirt road, or some other objectionable odour 
Respect, sir... ultra respect!
The more situations people want, I then look for a solution; whether or not I find one, I've learned things along the way. That's what it's about for me, understanding as much as I can about my truck as it's a keeper 
It could be the case, but it is idiotic IMO that the Ford engineers could not bring themselves to program it to light up, like it does in other cars. It only makes sense.
