Why do older, unused iPhones just die?

Thanks for the link, singh. I did see that article, and saw the part about charging to 50% every 6 months. I now know that's not a recommendation, but a must. Part of the reason I didn't get that from this article was the statement further above:

"If you store a device when its battery is fully discharged, the battery could fall into a deep discharge state, which renders it incapable of holding a charge."

That confirms the potential for deep, unfixable discharge of the battery. But how come, right there, they wouldn't add "The deeply discharged battery may also leak and destroy the rest of the phone's hardware," if that is a well-known fact?

To deggie:

Don't know how you concluded "troll," but everything I said was true and I really wanted a true reason why the phones failed, that's the only reason I posted. And yes, this has happened to myself and my wife a total of 3 times, once with an iPhone 4 and twice with two iPhone 6's. The old phones we were trying to reboot were always kept in very good condition, except for turning/leaving them off for up to 13 months, when we happened to hear from some friend who needed a working phone and we tried to reboot one of these to gift them. Unfortunately, in each case the non-bootable phones were too many generations back to qualify for anything but "We'll recycle it for you, but no credit," which is what I heard one more time, yesterday, at the Genius Bar in Santa Barbara, as I was ordering my new XsMax. Trolls don't order XsMax's, just to come ***** on this forum, so you can be sure I'm a real 4-decade dedicated Apple fan. Guess you're theory about leaky lithiums is accurate, and I've simply been unlucky (and foolish) enough to leave those 3 phones off just long enough to test, and prove, your theory.