Video cards require their own heat sinks and fans because the average video card these days uses more power at peak than a CPU. However, video cards these days

Video cards require their own heat sinks and fans because the average video card these days uses more power at peak than a CPU. However, video cards these days also tend to have the same sorts of power throttling that most cpu fans have these days, so tower boxes are a lot more quiet than they were a few years back. Also, there are a lot of different case sizes even in the area of "tower"s, with many of the higher end ones being very quiet even at full power. Also, for super-quiet computing, there are companies that make fanless power supplies now, or for those really concerned about silence, there are water cooling setups that use their big radiator, and a large, slow fan, to quietly cool the gpus and cpu in a modern system.