Why do wifi channels overlap?
Bandwidth is money. The more they can fit into a frequency space, the less it's going to cost. 

If it were done non-overlapping you might get marginal gains, but it was likely an engineering/cost tradeoff. At the fringes of the channel the amplitude of the signal will be lower due to imperfect filtering resulting in less noise travelling into the next channel anyways. Another reason this is done is because it's not likely to have more than 2-3 802.11b signals in any given locations so the other channels would just be there to try and avoid the other 1 or 2 in a given location. If you find a location with 4+ wifi signals, you'd likely make some gains by giving the channels more bandwidth, otherwise, you're just burning money by requiring more bandwidth.