Because the wisdom of that selection isn't so wise, now that there are almost 8 billion of us. Natural selection is dictated by conditions; conditions change. There is nothing permanent about it. And existing population of the target species (humans in this case) is itself one of the conditions.

Also it's probably a mistake to attribute something like wisdom to what is essentially a statistical crap-shoot. The great temptation of religion is of course the idea that there is an Intelligence at work. But just to pick a silly example, does the dodo bird or the passenger pigeon think it was "wise" and "intelligent" for the forces of nature to select for meat-eating humans? Dodo philosophers and theologians in the last days - did they question why God would unleash such a fury on them?

Anyway I can tell you human ones will. Natural selection will most likely start killing off humans as soon as our technology can't keep up with all our baby-birthin'. It will manifest in forms you're already seeing in today's headlines. Voluntarily consuming less is an attempt to deal with it before the wisdom of natural selection deals with it.