In 1990, Atari (the original Atari) were in bad shape. So they sold off their inventory of old cartridges. O'Shea Ltd bought them. All 3 million of them.
They bought them so they could sell them on to you, the public. And where do they keep these millions of Atari cartridges? Why, in a 20,000-square warehouse carved from a limestone cave, of course.
In the years since 1990, O'Shea have sold two million of the carts to customers around the world, leaving, yes, around one million of them left. There are carts for the 2600, 7800 and even a few for the Jag.
Best part? These being "new" carts, and O'Shea being specialists in the closeout business (ie selling crap after a company goes bust), the games are going for a flat rate of $5 each.
GameSetInterview: On Atari Cartridges In Deep Caves [GameSetWatch]