Because Stress keeps coming back.
Your mind does not like being stressed, and will do what it has to, to calm down. Junk food is high salt/sugar, which numbs things down a bit, to make it easier to deal with.
Since we cannot numb the situation, we numb ourselves.
Check it out sometime, see how you feel after eating junk.
Calm? Happy? Content?
Best Defense, take a walk, breath, or carry healthy comfort snacks.
I'm pulling for you!