Why isn't Voltage considered in mAh ratings?
There are two reasons why batteries are rated for capacity (A·h) instead of energy (W·h):


Cells using the same chemistry will have equal (or very close) voltage ratings, so voltage can be factored out when comparing their capacity.
In many chemistries, the voltage changes significantly during charge-discharge cycle, but also with temperature and load. Factoring that variable voltage in the battery energy will make many calculations complex:


a cell discharged to 50% of its energy would require e.g. 80% of its nominal energy to recharge back to 100%, while a cell discharged at 0% may require 200%. With capacity, charge and discharge figures are much closer, and the losses are nearly the same regardless of the charge level.
the same cell at 50% energy would gain some energy by simply being heated up. Its remaining capacity, on the other hand, would not change significantly.
the same battery would deliver 95% of its energy to load X, but only 70% to load Y. This is also true for the capacity in some chemistries, but to a lesser extent.



As you can see, if you say "I have a batter of 1 W·h", you'll need to specify in which conditions this energy has to be consumed, while "a battery of 1 A·h" characterizes the battery itself, not the environment where it will be used.