Probably all those routers are clustering on channel 11 because that was the default set at the factory and their users are totally unaware there are different WiFi channel much less how to change their router channel assignment.
iirc, channels 1, 6, and 11 are the common default channels. There is some 'interference overlap" in the 11 channels, and only 1, 6, and 11 (the lower and upper, and the middle channels) can be used simultaneously without interfering in any way with each other. The closer you get, the more interference you get. I think Linksys uses ch6 on all of their products, and given how popular they are, getting OFF that channel alone can solve a big chunk of the wifi problems.
