With the environment changing so do the evolutionary advantages that come with the environment. In that regard, an evolutionary trait that is considered disadvantageous in one environment might be actually advantageous in a different environment.

Case in point: Humans fighting over rocks.

When there were only a few humans around but lots of "good rocks" to fight over, being competitive for the few humans might have been an advantage. Because there had been enough "good rocks" around so everybody could get their own piece of rock if they compete and some can even get more than just one rock.

Now change the ratio between humans and "good rocks", whether that be because there are a lot more humans than there used to be or because there are far fewer "good rocks" around to compete for or simply a combination of both.

In such a scenario competition for the sole ownership these few remaining "good rocks" between big numbers of humans, might be a massive resource waste compared to a more cooperative approach that doesn't pit everybody against each other for a scarce resource.