

Banks and corporations change computers every 3-5 years because accounting love to lease rather to buy
3-5 years is a pretty standard depreciation schedule for IT equipment like computers, peripheral accessories etc.
Computers and laptops (using Straight-line method): 31.67% with a useful life of 3 years.
Computers and laptops (using Written Down Value method): 63.16% with a useful life of 3 years
It really has nothing to do with leasing vs. buying.
I can’t speak to your specific examples since I don’t work there.
The reasons beyond CapEx considerations are things like security, compliance, warranty coverage expiration, standardization across the org, general employee satisfaction, hardware falling out of vendor support.
I doubt the banks computers are single purpose or purchased specifically for each job role. Sure a 15 or 30 year old computer might technically work but there’s no way it’ll meet regulatory security compliance rules.
The home user/hobbyist approach really doesn’t scale to corporate IT.