Lithium-ion and lead-acid battery technologies are very different. A lead-acid battery performs extremely well so long as power keeps flowing through it - my first car was 12 years old when written off and had its original battery, because it was driven often and kept charged.

Lithium-based batteries suffer chemical degradation of their electrodes related to time as well as charge cycles. Even an unused lithium-ion battery can die before it's ever installed in a product. The electrode damage is what causes the capacity to decline.

For the record, lead-acid batteries can suffer similar degradation due to sulphur build-up on their internal plates, but regular use keeps this at bay and a well-maintained battery will just keep working. Part of the problem is that it's nearly impossible to maintain modern batteries as they're all sealed by design.