Except that they're not needed to get to small and lightweight. Those qualities are driven by the size of components decreasing due to die improvements. So they can stick smaller, better components onto boards every year.
Look at enough device tear downs, and you realize that lack of repairability is more about laziness than it is about legitimate tradeoffs.
There are physical limitations on socket sizes and construction that can allow owners to reasonably upgrade the equipment without damaging it. There are design limitations on arranging components in such a way that they can be accessed. Also the majority of users never open their machine; why should they have to pay more so the privileged few can pay a little less for storage and RAM?
As someone who has recently assembled a commodity PC (Mini-ITX) and someone who has fiddled with RPi hardware, I can see the disadvantages facing a consumer hardware manufacturer attempting to satisfy a tinkerer's urges. Notebooks, all-in-ones, and Mac Mini-sized desktops are just at the edge of serviceability, and clearly the mobile and tablet product lines are well beyond it.
It's not just tinkerers. If you crack your screen, the glass is generally glued onto the screen, necessitating replacement of the entire display assembly, which isn't all that much cheaper than just buying a new one. They do this on purpose to force people to buy more devices.
The only reason they get away with it because we don't force them to take repairability seriously. As someone noted, most customers prefer new devices.
You only need to purchase more devices if you make it a habit to break the ones you have. No one's being forced to do anything in the repairability case.
Look at enough device tear downs, and you realize that lack of repairability is more about laziness than it is about legitimate tradeoffs.