It is about the money, Apple would rather you just buy a new machine than fix or upgrade what you got. That's the bottom line.
Example, the 2018 Mac Mini has soldered storage, no way to upgrade or replace it if it fails, without replacing the entire logicboard anyway. The entire machine, which could be working just fine, would be a write off because they couldn't be bothered to put in a 4x PCI-E M.2 slot. Of course, Apple wouldn't want users to know that third party NVMe drives are way faster and cheaper than what they are shipping, so I can see why.