Probably because a lot of people want to cruise in the 14, 15, or 16 while in the middle chainwheel/ring and there will be a lot of cross-chaining or wearing out of the outer cogs/chain. Shouldn't be a big deal if the chainlines are close to ideal and they are using the cogs that best match up, chainline wise, with whatever the front chainwheel/ring (for example: outer 3 cogs with the largest chainwheel/ring). Seen a lot of people with 9 to 10 speed cassettes and a single chainwheel and they shift without any problems and haven't heard them complain about wearing out the cassette or chain sooner than they expect or the chain jumping under power pedaling.