BT partly right
Wireless congestion in urban areas is caused by ISPs like BT giving out wireless routers like candy.
Where I live, surrounded by young singles and couples in bedsits, it isn't uncommon for me to detect anything up to 30 SSIDs -- no doubt located within feet of the laptops etc to which they are connected by wireless.
Perhaps typically, a friend's tenant had his desktop machine within inches of a wireless router, where it could have been connected by cable for better results (and less interference for neighbours).
The situation is getting critical with the advent of smartphones, as users can save by connecting via router wireless rather than 3G -- and tablets which only have wireless connection (which is usually always on, no doubt adding to the problem).
The ISPs don't help by setting the routers to automatically detect the "best" channel -- which I guess just means they avoid vying with the strongest channels nearby. This doesn't help when they are nearer my PC than our router and switch to and interfere with "our" channel (the one I found gave best reception for me).
Have had to daisychain a second wireless router -- which boosts signal -- but inevitably adds to local congestion.
The choices made when the current system was agreed must have been based on typical suburban usage and no expectation of today's popular hardware. The interference overlaps between all but three of the (up to) 13 channels means the system is approaching unusability in urban areas today.