That's because they weren't working as hard, and certainly not 24/7 (except for the clock if you had one).Speaking of age - batteries used to last for a decade. Now, three years is a decent clip. I had to decommission a beautiful Odissey after only 5 years - it will not charge beyond 12.2 V.
Using a smart charger that conditions the battery greatly extends battery life..
The other thing is to buy a quality battery. Many different labels , but they often are the same low price battery under the skin.