Battery and volume indicators are the bane of my existence—they never work as expected! Sure, there are complicated issues at work such as logarithmic scales and perception, but those excuses don't ease my headache. [Reddit via Geekosystem]
Battery decay is not consistent over the life of the charge. It actually starts to decay quickly and then tails off. If you have a scale like you asked for then your battery meter would go crashing from 5 to 2 in a very short amount of time and then take a very long time to drop to 1.
In other words, it would be completely useless and make no sense.
The reason that the scales are calculated in the way you hate is precisely to make it appear like the battery decay is constant - in other words to help you better judge the life of the battery.
Trust me, if it was as easy as you think it was to display battery strength, there wouldn't be a need for people to fudge it in this way :)