So it's widely known as a fact that many Nintendo games have ridiculous legs. But here's the question that no one really bother to ask...why is this so?

[Mummelmann]

Timeless gameplay that doesn't live off of trends and current gimmicks as a main sales incentive. That's also the reason why Gran Turismo has such great legs and games like Baldur's Gate can be re-released and still do well.