Convergent evolution is the answer you are looking for. An example of convergent evolution at work is both Dolphins, and Sharks having dorsal fins, and powerful tails. Even though Dolphins are mammals evolution has selected for them to have dorsal fins, because it is advantageous. The same goes for why birds, insects, and bats all evolved wings independently. Being able to fly is a huge advantage and having wings is the best way to get there.
Taking Convergent evolution to the world of Splatoon, we can say that since Octolings and Inklings both started raising and caring for their young, evolution selected for them to have mammary glands. (Yes, I realize Inklings don't have nipples, but that doesn't disprove the young rearing hypothesis at all. Why? Because evolution is a slow process, and we are likely seeing Inklings in the middle of the process of evolving complete mammary glands, with nipples and all.)
