The batteries have a hard life as they are often installed in tractors that don't have a good regulator or in some newer tractors they don't even have a regulator, just a diode in series with the charge coil. They also get left out in the cold and unused for months at a time and loose their charge then bake inside a hot engine compartment in the summer. I figure if you get more than 5 years out of one it's done it's duty.
I think that's the big reason, they don't get used and go dead, then get recharged in the spring. Lettin g abattery completely go dead and sit shortens the life span. Not to mention, they have smaller amperages and if they get any loads on them, they can't handle it like a car battery would. I have noticed the batteries in the tractors I tend to use all year round last much longer. I just replaced an Autozone GT battery after 8 yrs, this tractor was mower duty in summer, snow blower in winter, it was regularly charged and used.
Shoot, I just had to replace the top of the line HD battery in the wife's Jeep Liberty after only 2 yrs, it wouldn't keep a charge. Glad I had a 3 yr full replacement!