[dc2447]

There are a few factors here.

Depreciation: is typically three years for hardware purchased rather than leased. At the end of 3 years the item has zero value thus can be replaced.

Useful life: is hopefully longer and up to five years. So an item is without value but can still be used. More typical with networking hardware than compute.

The other poster who mentioned power consumption is totally correct. It can make sense to renew hardware even before it has fully depreciated in order to get better datacentre density and lower power draw.

This is what is driving most refreshes in my experience.