Quoted from Pin_Guy:
inadequate storage environment with large temperature swings, dampness, or excessive temperatures.
I agree--storage conditions certainly play a big part. But batteries are still batteries, even if stored under perfect conditions.
The lifespan is typically 7-12 years for coin cell batteries, although it's usually just stated as 10 years. Depending on the current draw, they could go dead before then, but that doesn't necessarily also mean they will leak after they are drained, or would leak immediately after the 10 year mark.
System 3 games were made between 1989 and 1996. That's 22-29 years, depending on the age of the game, plus whatever amount of time the batteries were sitting on a shelf. That's well past their typical lifespan.
I've replaced many coin cell batteries on system 3 MPUs in the past few years and have had to clean up after the mess they made (not nearly as bad as alkaline, but still requires some clean-up and the affected components to be replaced). Like I said, batteries are batteries, and it's better to replace them as a matter of preventative maintenance rather than wait for a problem to develop, especially if they are 20+ years old.