Today in Learning (tm) with Real Actual Facts (tm):
2.4 GHz wifi has channels 1-11, but really only 3 non-overlapping 20 MHz channels, generally agreed to be 1, 6, and 11. (Though some troublemakers apparently prefer to use 5 instead of 6.) The existence of the other in-between channels serves mainly to confuse, since if your router uses an overlapping-but-not-aligned channel, you probably won't see its carrier, but it will still happily interfere with you like crazy.
5 GHz wifi is known to have "more channels", but how many more? Well, there are at least 21 channels licensed for use in the U.S. But here's the catch: they're all 20 MHz wide instead of 5 Mhz! So that's not just twice as many, that's 7x as many. And as a bonus, the wifi standards people got it right and disallowed in-between channels, so you don't have weird overlapping interference like in 2.4 GHz.
As a double bonus, 5 GHz gets blocked better by walls. That may not sound like a bonus, but when it comes to your neighbours not interfering with your signal, it definitely is.
No wonder 5 GHz is quieter.