Why do Batteries go bad?

Seems like keeping them fully charged, in a not overly hot environment, slows down the degradation of the plates (after all, the SO4 isn't going to fall out of solution & short anything out if the battery is fully charged (or even being fed a small current with a maintainer). Along with topping them off with distilled water, of course. At least it generally works for me.
I buy high CCA battery of the largest physical size that will fit the battery box.
I do the same thing. Last week I put a new series 34 battery with 750 CCA in my '97 Ford Escort with a 2.0L engine. Even doing this I often end up with the battery not lasting through the warranty period I think the best battery I ever had was an Interstate lawn & garden battery that lasted about 8 years.
SULFATION!! setting in a low state of charge kills batteries. years ago i got diehards from a local sears catalog store 2 BAD batteries because they set on the shelf being an odd size for my 62 chevy belair were + discharged beyond use + each only lasted a few months, both warrantied + money refunded after returning the 2nd one!!
Two batteries, of the same size, but with different CCA ratings...the lower rated one will have fewer plates, usually further apart (separated). Most batteries will die from sulfation. This is from the plates shorting out...if the plates are further apart, they can be more resistant to shorting from sulfation. So yes, high CCA batteries can often have shorter life as the manufacturer packs more plates in the same size box.
Not necessarily, could have same # of plates, not further apart, just less lead and surface area on each plate.

This also reflects the real world situation that the cheaper lower CCA battery doesn't have a longer lifespan unless it is lower CCA because it uses a different plate construction because it's a deep discharge or hybrid instead of starting battery. Then it will cost more for the lower CCA battery than the equivalent starter battery of anywhere near the same CCA rating.

The thing to remember is that battery manufacturers are not looking to sell a much cheaper battery without making it much cheaper, the most cost effective CCA to BOM ratio possible. This is achieved by maximizing the surface area of the lead used, making thinner plate lattice structures, at least as susceptible to sulfation which doesn't necessarily short them out, just blocks active regions of chemical reaction.

Plus, it is seldom that I have a battery die from shorting out, rather that it drops below minimum required CCA. Start out with a lower CCA, and discharge it deeper every time you start the vehicle because it has a lower CCA, and it falls below the minimum needed CCA sooner. My automotive battery replacements almost always coincide to not providing enough CCA in winter, for anything driven often enough to keep the battery at a good SOC.

Ultimately, I don't see what all the fuss is about. I usually get 6 years on a ~$100 battery. If the cost per year differs by little more than the cost of lunch (if that), it was a waste to bother fixating on it within the context of TCO of a vehicle.

The more common reason that batteries go bad prematurely in this era, is manufacturers trying to use smart charging algorithms to eek out slightly higher fuel economy so the battery perpetually stays at a lower SOC than they used to, in addition to the higher parasitic drain of modern vehicles.

Case in point: Two SUVs I have. One is much older, uses an old school charging setup, gets 6-7 years out of a battery even though it is almost exclusively short-tripped. The other uses a modern smart charge logic, is always driven longer distances, stays in the garage where it doesn't crank long in winter, and gets about 2 years shorter life out of the same make/model/group-size battery.
I personally pay more attention to RC (reserve capacity) than I do CCA. Two batteries of the same weight and size, the one with lower CCA but higher RC would be my pick.

But then again I live in a fairly mild climate, I'd probably go the max CCA route if I lived in Canada.