Building your own gaming machine was always the best option if you knew about new technologies, compatibilities, brands etc. The problem I see these days is that the market is really, really saturated in everything PC. Which makes the research necessary extensive and time consuming for people who are not exactly “on the pulse” when it comes to hardware.
So it also becomes a question of “do I want to spend the time to get exactly what I need for the cheapest possible price?” versus just checking some meta-sites that review prebuilt PCs and pick one that is rated good by the community instead.