Barebones laptops are flogged to OEM's in vast numbers, only a tiny ammount are ever sold to Joe Public as OEM kit for a self build. Look at the Alienware chassis for example. The underlying chassis and hardware is sold to Dell, Alienware, Rock and can even be found on occasion in 'low end' kit sold by well known high street stores.
Problems off the top of my head:
They are not customer friendly to upgrade - while my GF could build a PC with some simple instructions your average laptop is not as easy to work on, spaces are tight, tollerances are small and they don't lend themselves well to being slapped together in a non methodical manner (anyone can - and regularly do - build a PC, some build them significantly better than others but then again your average hardware monkey has probably been doing it for 20 years by now....
You then have the power issues... While most laptops have socketed CPU's the coolers used are often quite limited in that they are spec'd for the generation of chip they ship with and nothing more, you don't have the space to add a bigger one or the power to put a faster fan on, you then don't have the room to put a larger fan into the chassis... you're stuck. Graphics card & CPU upgrades regularly require a better laptop PSU and depending on the design a power board (mostly older hardware to be fair). Then we get to screens and it gets a whole lot worse. Replacing a laptop screen isn't the chore it used to be but it's still not exactly what most would call fun. Then you have the warranty issues, a lot of people have access to a 2nd pc be it a collection of older parts that has been recycled as a 2nd rig or a friend who can test something for them. On a laptop that's significantly harder. In the event of a warranty issue or incompatibility issue your graphics card provider will point you to your main board supplier, they'll point you to your PSU supplier and they'll say you should have confirmed the power requirements prior to purchasing and refuse to do anything other than sell you new hardware as it's not a fault.
When you buy a laptop you are buying tested and certified hardware that is known to work together with a minimum of fuss. It will work or the OEM in question will repair/replace it FOC. That's what you are paying for, and they can do it a lot cheaper/better than you or I can.
