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...

Maybe it is just my ignorance, but that type of consolidation seems rare in any market in Japan, not just gaming.

I would say that if there is anything Japanese, and many other asian big corporations, are known for, is consolidation.

Samsung, Toyota, Hyundai, Sony... They all are huge conglomerates spawning across multiple industries.

They aren't really consolidated so much as they're interlocked. Many of the largest companies in Japan own stock in all of the other largest companies in Japan. It diversifies their holdings and insulates them from market fluctuations while maintaining their independence.

I was thinking of those too, but did they get that way via acquisitions or by entering new markets?

It's really a bit of everything. Some like Fuji, Hyundai, or Toyota, I believe have been historically diversifying across several different markets.

Sony did expand on some fronts via acquisitions, e.g. Sony Electronics acquiring Konica-Minolta, Sony Electronic Entertainment acquiring several studios, etc.