Three words, Puckton.
ROLL
YOUR
OWN.
It can seem like a big step for a first-timer, but the freedom of being able to swap parts out as you need more power, memory, or storage down the road is heady indeed. With the current component generation, there is almost no tweaking needed to get your system running, and when the question of "Well, what kind of X do I have" arises for support, you just go grab the box from the basement.
Plus, why pay someone extra for last year's technology, just because they turned the screws?