I thought it was mostly to do with warranties and accounting?
Our company religiously swaps kit out as soon as the manufacturer's warranty expires, presumably since after that point in time any kind or problem/repair is going to take forever and cost a fortune, whereas with current kit getting it fixed is relatively quick and cheap.
It also doesn't hurt that The Accountants classify stuff as fully depreciated and officially worthless after 3 years.
Having said that, I've seen someone trying to persuade a four year old laptop to run XP and all the endless layers of security and management crap that big companies love to shovel into the system tray. It took five minutes to start up or to shut down, would infallibly lock up if you tried to hibernate it, and had a battery life of about an hour. Ouch.


