Why the phone batteries degrade over time?

Phone batteries degrade because lithium-ion cells slowly lose their ability to hold and move charge after many cycles. Heat, fast charging, deep drains, and leaving the phone at 100% for long periods can speed that up. It is not usually one dramatic failure; it is more like tiny wear adding up every day. I think of it like comparing interstate vs diehard https://help-center.pissedconsumer.com/duralast-vs-interstate-batteries-which-is-better/ for car batteries: people look at capacity, See More reliability, and how the battery handles real use over time. For phones, keeping charge roughly between 20% and 80% and avoiding heat can help the battery age more gracefully.

Because they could get uncalibrated or destroyed by the owners, they can also run out of batteries overtime, so yeah that's probably the reason why.