You're right,
Apple started this with its excellent iMac computer (been out several years now, in various forms).
It does use some laptop components. Due to size constraints, it's also harder to upgrade than larger desktop computers.
But how many people actually upgrade their systems? The answer is not many. So for the vast majority of people (regular computer users), iMac-style designs are great, because they save space and look nicer and neater.
The most common upgrade is probably RAM, which is still very easy on iMac-like designs.
Some of them can be mounted on the wall, yes. The mount standard is called VESA.