The problem is, there is no generic standard even within one company. The motherboard will be a different shape/size depending on where the battery is, how the RAM fits. Some laptops even run on different voltages. Even the power jacks aren't standard! Screen sizes are different, the cable routes in a different way, the input on the monitor itself is often in a different place so you have to buy a very specific model of replacement screen.
That's all before you take into account cooling, which works differently in practically every laptop model, heatsinks are different sizes, GPUs and CPUs may be in one chip or two, further apart or closer together, run hotter or colder. Chips in laptops tend to run hotter, so you have less movement in the size of the heatsink etc.
Totally different to buying a big box and sticking bits in it really (desktop), hell, somebody I know built his PC and pretty much plopped the bits into a big fish tank, and filled it with oil for cooling. You just don't get that flexibility with a laptop!
Some laptops do have removable CPUs, most have removable RAM, most have removable HDDs, some have upgrade screens available, few have upgrade graphics cards available (often alienware etc, not integrated cards etc). That's the best we're going to get with laptops I think!