Many species are not cannibals, humans (usually) included. But why is that?
Ignoring humans for the moment, one of the reasons most animals avoid cannibalism is that whatever parasites or other diseases may be infecting the eaten animal will easily infect the cannibal since they are the same species. A parasite or disease evolved to a specific host species will naturally already be evolved to infect any cannibal of the same species. But they won't be as capable of infecting a predator of a very different species. So, basically, cannibals tend to acquire any sicknesses their victims have. Recent research has confirmed this hypothesis: http://biology.cos.ucf.edu/hoffman/wp-content/uploads/2013/09/PfennigHoHoffman1998.pdf
The same is true for humans: Human cannibals tend to acquire various diseases, a prominent example being the disease called Kuru (https://en.wikipedia.org/wiki/Kuru_%28disease%29
) which is kinda like Mad Cow Disease for humans (though Mad Cow Disease is also dangerous for humans, too, not just cows).
So, the basic explanation is an evolutionary one: a lot of the time, being a cannibal is bad for you.
On the other hand, in many species, cannibalism can be good for the cannibal, especially under certain kinds of circumstances. For example, in some species of insects (such as the preying mantis) and spiders (famously the black widow spider), the female eats the male after mating, as a kind of macabre 'wedding dinner'. There are other examples of cannibalism being an evolutionary 'good thing' at https://en.wikipedia.org/wiki/Cannibalism_%28zoology%29
In humans, however, in *theory*, you could avoid most diseases by cooking. Kuru and other prion-based diseases are notoriously difficult to destroy by simple heat, though. However, if you *could* avoid something like Kuru, then *in theory*, you could make a case that some forms of human cannibalism could 'make sense' as you do in the OP.
But when you get down to it, I believe that we probably have an innate revulsion to cannibalism, which would come from our long evolutionary history *before* cooking with fire was available to us, which was only in the neighbourhood of about 200,000 years ago (https://en.wikipedia.org/wiki/Control_of_fire_by_early_humans
), which is a very short time in terms of evolution.
Some cultures, in certain conditions, have overcome this revulsion, and cannibalism is accepted (usually not for regular meals, though, more of a ritualistic kind of thing). But these are relatively rare.
Another thing that we probably have an innate revulsion to is incest, which is also known to cause disease (genetic diseases in offspring), and which many other animals also avoid. Similar logic applies as to why we might have this revulsion.
But also by a similar logic, you might be able to make a case that, like cooking for cannibalism, use of birth control could make incest 'make sense' in some circumstances.
Not to say that making these arguments won't earn you some scornful responses and/or shunning, though.