Why people like to see pornographic scenes, but do not feel the same about eating scenes?
For these two phenomena to create the same feelings all of the time, the physiology that engenders these feelings would have to be the same.  All I can do is point out that these have been separate and distinct even in yeast and bacteria and invoke a likelihood argument. 

The impulse for hunger is almost as old as living things.  In bacteria, cyclic Adenosine mono phosphate (cAMP) mediates hunger, though this does not appear to be universal.  

Sexual reproduction appeared much later - in the single celled eukaryotes mating factors for instance are mediated by the MATa/alpha receptors.  

In animals like C elegans the differences persist (hunger, mating).  This is not a precise argument, but there is the entropy argument - the odds of these 2 signals evolving together biochemically or genetically is quite unlikely given how separate they have always been. 

That's not to say that eating food can't create a sexual arousal, but it does not do so all the time.  The sexual response appears to be plastic and adaptable to nearly any sort of stimulus.  The fact that in primates and many other animals, the pleasure stimulus is related to both mating and eating might make them seem like similar impulses, but when you go deeper it doesn't seem plausible.  So the two could hypothetically create the same feeling, but if you are asking why doesn't it usually... then there are lots of reasons why.  

I find it helpful to reverse the question and ask - what would happen to a species where you could suppress the mating impulse by eating?  Probably not an adaptation that would survive long.