With elementary math, consumers could easily calculate battery life by dividing watt-hours by power consumption.
No, that's not true at all. That's not how computers work, and that's not how batteries work. Battery capacity is not like a spring, where force in and force out are equal. The amount of power that it takes to charge a battery differs depending on how you do it, and the amount of power that you get out also depends on how you draw it.
In general, the slower you discharge a chemical storage battery, the more power you can get out. (The same is mostly true of charging them, but there is more nuance.) The computer does not draw the same amount of power all the time. Exactly what consumption will be differs based on what you run on it, and how you configure the settings.
The sensible thing to do is to expect battery life ratings in best, worst, and typical scenarios. I'd also like to know how long a device will idle and sleep, if it does those things.