That's because the Japanese game companies are more or less in friendly coopetition with each other. Both Namco and Sega run game centers (arcades), which means they're buying each other's games to populate said centers (as well as other manufacturers' games). And then there's Smash Bros., in which many of Nintendo's competitors (including Microsoft -- twice) went to Nintendo and said, "hey, could you feature our characters too?" And then there's Mario & Sonic at the Olympics...