A:
Because internet is USA-centric. In USA, use of channels 12 and 13 is discouraged and the remaining frequency bandwidth is most uniformly used in 1-6-11 layout. Plus it allows for proper guard band between channels if one operates 802.11b (does anybody still do it? IMO it should be outright banned by now).
Elsewhere on the Globe, where channels 12 and 13 are not restricted, using 1-5-9-13 gives 33% of performance increase as it allows 4 non-overlapping channels (only guard bands overlap). Of course channel layout 1-5-9 is apllicable also in USA, but until OP identified the problem with WiFi drowning ZigBee nobody in the States saw any need to use frequency band efficiently. But then, drowning others with WiFi isn’t seen as a problem by most of WiFi users, bitching about DFS prooves that.