Why?
Because, as far as I know, our species would not have survived so successfully if it had not had such a widely varied diet, and because there were times when plants and fruit would not have been available, whereas people tended to live almost exclusively - and again as far as I know - by water, so that they had fish to sustain them.
Do you think our species would have evolved as we are without meat being included in the diet of our ancestor ape and , therefore, inherited by us via natural selection and random mutations resulting in what we are?