@gordoooo_z because WiFi channels overlap. The distance between channel 1 and channel 2 is less than the width of channel 1. The only way to fit three non overlapping channels in the 2.4 GHz allocation for 802.11b/g/n is to use channels 1, 6, and 11 like you see here.
Follow
@uberduck @gordoooo_z
They also used to be narrower — first they were 20 MHz bins, later 40 MHz and now they are 80 MHz.
When they are this wide, it's indeed impossible to fit more, considering that some frequencies are reserved in some countries. There is a way to lift these limitations, but not all equipment might support this.
@m0xee @gordoooo_z you're correct for the 5.8GHz band, but the diagram shown in OP's picture is 2.4GHz with 22MHz-wide channels. Each channel is separated from its neighbors by 5MHz, so each channel interferes with its two neighbors on each side.