My noob perspective is that generation periods are optimized for environmental change sensitivity.

Humans seem to reliably be able to handle environmental changes expected in their life spans. Other species not, and I dare say most other fauna has much a faster generation iteration rate because they are much more sensitive to the environment. Ergo evolution made their lifespans shorter so they can iterate faster and respond with a tighter gene fit to the environment.

Humans have their fitness less correlated by genes so a longer life span is allowed, but not boundless. In a sense every body must die so that new solutions/iterations can be ran.