If it were not the favorable strategy, it wouldn't have to be illegal, because it would not be popular enough to hurt consumers. People habitually favor optimizing for short-term success because there is no long-term if you don't survive the short-term. Furthermore, it is actually repeated interaction that makes collusion/cooperation better than betraying, because of the opportunity to get punished on subsequent iterations. It is this medium-term thinking that pushes collusion (but not too much of it) into a position of dominance. In the truly long term, nothing is stable because we're all dead anyway.
Collusion is illegal because it harms consumers, not because collusion is the "default strategy".
In fact, collusion is not the default strategy simply because it is not stable in the long term; there's a lot of research which demonstrates this.