Why is there a 5 year time interval replacement recommendation for spark plugs? What happens to spark plugs over time if there is very low mileage?

For example, my 06 C55 has 24K miles. It's a quarter of the 100K miles recommendation, but it is 5 years.

Very curious why I should be changing the spark plugs.

 

I changed mine at 100K and they looked perfect still. I can't think of any reason to change them at 24K. There is no wear and tear happening unless you're idling the car for hours a day but not driving it. They really should put an hour meter on cars like they do on most trucks. Thinks like plugs need to be changed based on engine run time, not miles or years.

 

That's what I thought. Maybe the 5 year comes from an estimate that your engine is running somewhat longer than the actual mileage you're doing, so for someone with close to 100k miles vs 5 years, I can see how that would be true.

Thanks.