Well, my answer is big fans (so greater airflow at any given RPM) designed to minimise noise (blade shape/angle, bearings) on shock absorbing mounts (minimising the amount vibration is transmitted to the case) controlled intelligently based on motherboard and CPU temperatures.
Each fan has its own ramp up/down profile, the case is insulated, the closed system watercooling on the CPU has a very very quiet pump and although at full pelt the case can probably take off and hover, under normal and even normal gaming use I can't hear a thing from three feet away.
When it's 30C outside (and I don't have aircon) and I'm stressing the circuitry for extended periods the fans have the capacity and ability to keep the system well within operational temperatures and I do get some noise, but it's nice being able to cope with that abnormal scenario and still have a silent system the rest of the time.
It does come at a cost, but that comes to around 5-7% of the total system price. I could understand people preferring to save that cash, I find it valuable.