And it's hardly the only reason. The most prime reason is that people think that more = better. If I have a higher number on my PSU, my FPS will increase.
Since it's the only number they come across that make sense to them, they incorrectly assume that having more is a good thing.
A good PSU won't have loud fans either way. The X-series from SeaSonic for instance, have fans that barely runs at all until 75% load, and only in high ambient temperatures. At 100% load, I think the fan runs at 33% or something like it.