pretty much everything is strapped to the board to start with
more standards in general would be a start!
Virtaully every range of laptops has its own unique motherboard, cooling, chassis, socket layout etc, theres not really a standard like in desktops (mATX, ATX etc.)
you can buy mobile cpus, laptop gfx cards and mini pc cards its just that not many places stock them and theyre still expenisve because of this
as said OCUK do custom build laptops
You can buy the shell and internals if you really want to, mainly from trade suppliers but adding up the prices it is usually more than buying a pre-built machine and you've potentially got the additional hassle of trying to calculate the thermal envelope yourself if you want to ensure parts aren't liable to overheat.
I'd suggest it simply isn't worth it for most people and you're still left with the same upgadability problems of a pre-built in most cases.