There are a few reasons.
The purpose of a wide spread of gears is efficiency over a wide range of speeds. The type of customer that regularly achieves very high speeds - those that regularly cruise the German Autobahnen - tends to choose an automated transmission over a manual. Those that choose manuals tend also to choose slower cars, so there is simply less need for a wide spread of gears, so a smaller number of gears can cover the required spread.
The purpose of more speeds is increased efficiency, but only if the shift times can be kept short. Each delay in shifting is an efficiency loss. With certain automated transmission, either a double-clutch or a more traditional epicyclic torque-converter auto, shift times are short, so shifting losses are low. With a manual, the shift time is down to the driver, and is often relatively slow, so the shifting losses start to outweigh the efficiency gains of more speeds, whether you are measuring fuel consumption or acceleration times.
The choice of manual over automatic is often down to cost. Each extra speed adds cost, reducing the reason for choosing a manual.
There is limited space in the gear selector mechanism for adding extra gears. Putting more gears in the limited movement of the lever makes correct gear selection more difficult, which both reduces efficiency, and reduces driver satisfaction.