If I had to buy a laptop, I would buy a Dell. I don't even know if it's possible to build your own laptop, and I sure wouldn't want to try.

I'm not going to knock Dell, I just used them as an example. The have great PCs. I could have just as easily said Hewlett Packard.
No matter what though, you pay a HUGE markup buying an off-the-rack PC, which was what my point was. And you often end up with  parts that can't be easily upgradable, such as a motherboard that can't accommodate a standard Intel processor, or a case with proprietary specs that won't accommodate standard motherboard sizes.