Not in electronics. Wh is a pretty difficult metric in electronics design (and thus, speccing the time a battery will last).

I have said this before because all ICs use mA as their power consumption rating because they might have a range of 1.8V-3.6V of operation or 4.5-10V or something and they consume about the same amount of current across that spectrum but vary in power. This is why low power systems often use 1.8V.

Batteries also vary in delivered power at a constant load. They can pull 100mA continuously, but a lithium ion cell delivers 420mW first and then continually falls until it delivers only 250mW, almost half as much.

What is easier to calculate?

• Integrating across a variable voltage domain for the source and then subtracting each component uses variable power, then integrating each component over its voltage range. Oh and the battery capacity left in wh is also nonlinear, so when estimating state of charge, you have to balance a nonlinear source with a differently nonlinear load, integrated over time, all on a 200MHz mcu trying to do 50 other, more important things (and that's fast)
• battery can deliver this set current for this time, circuit pulls this amount of current, battery lasts X hours. Estimating life cheaply is just:

"get starting SoC from memory and voltage" "measure current once" "measure current again" Current*time=mAh used. Save

Again, not saying it is "correct", but significantly easier on all levels.

For the consumer. Why does it matter? There is absolutely no specs every given for actual power used. Does your phone use 1W or 5W or 100mW on average? Never given.

Batteries are literally just "bigger = better". Using Wh instead mAh would not change this at all. The only thing it would do is expose the 1% that try to fudge the numbers while everyone else just fudges power consumption.

Oh you got X phone because Y phone only had a 13Wh battery instead of 18Wh. Oh too bad, phone X uses an average of 9W and only lasts for 2 hours. Phone Y used 0.5W.