Long ago, it was *much* cheaper to build than buy. Then Dell came. Dell sold computers for below the retail cost of the components. It became cheaper to buy. It has since remained cheaper to buy, if you match a pre-built system. It's cheaper to build if you spec a system that nobody sells. Systems rarely have vastly different level components. I built myself a gaming rig. It was built to compete with a friend's I spec'ed the best gaming video card for the budget, and did everything else as cheap as possible (while still of acceptable quality). The result was a computer $200 less than my friend's brand new computer, with better FPS for every game we tried. It was much slower at video encoding, but played games better.
Bought computers are hard to get anything that's not "cheap" "middle" "workstation" or "gamer". You can't have a gamer card in a cheap system, or vice versa. Building is good for flexibility, and picking components. Another time I built, I saw the CPUs as having a poor bang per buck, so I built a good system with the cheapest CPU I could find. 2 years later, when the system was pretty bad, I upgraded the CPU and had what would have been a top-end (out of my price range) computer for budget price, though a little late.
So the decision is personal and fluid, though many here think everyone should do as they do, rather than think for themselves.