A standardized form-factor allows easy replacement of motherboard, CPUs, screens, keyboards. Desktop PCs still use ATX cases, ATX power supplies, ATX motherboards.
Desktop PCs can be upgraded while keeping the monitor, keyboard and mouse: the entirety of a high-end gaming PC upgrade can consists of a new motherboard, CPU, RAM and GPU.
There's little reason laptops couldn't have went down the standardization path in the late 1990s. The chassis of a 1998 IBM ThinkPad should be able to components from 2019 without much effort. Just like an ATX desktop case.
Funny you mention that - my MacBook Pro was repeatedly serviced during my 3 year AppleCare period, and I noticed that instead of my warranty status showing AppleCare on the statements, after the first couple of services that status had changed to "Australian Consumer Law Requirement".
They still didn't provide me with a replacement machine though - they just keep replacing the SATA flex cable every few months. I'm switching to a ThinkPad now.
The only thing holding it back is I could not for the life of me get Boot Camp working with Win10 — UEFI nonsense.
The new Air is finally actually tempting me to upgrade. Before that there was nothing I thought was worth considering.
That's not a very long time. Business laptops' extended warranty is 5, and to be honest, 5-6 years old laptops - actually, nearly anything since modern sleep and aes-ni - are perfectly enough, so instead of buying new, it really should be possible to fix a hinge, a display, a keyboard once in a while, instead of needing to buy a new machine.