Why is no capacity indication on regular batteries?
The answer is: marketing. Some number, like "1800 mAh", doesn't really mean anything, anyway. At what temperature? At what voltage is the battery considered "dead"? At what current? The charge you can get from any type of battery really depends on these factors, and how far dead you are willing to run it.

All the major alkaline battery manufacturers supply datasheets that specify the capacity of their batteries more completely. This is the place to get capacity information, not the product label.


Energizer
Duracell