In addition to being more expensive, you don't really know the motherboards will be better. I can go get a motherboard from a variety of sources from companies that nobody's heard of because they started up last year. You have to trust that your technicians aren't just doing that. That's not the main issue though. Self-building works fine for desktops, but a lot of buyers and users don't want desktops. Laptops have a lot of convenience, and now that their processors aren't hampered compared to the needed office workloads, they're quite suitable. I have rarely seen a successful self-built laptop, let alone one that people would really want to carry.