That's a bit cursed mental model tbf... The voltages of batteries, in the first place, is function of state of charge. 100% = 4.2V, 0% = ~2.7V, 50% is 3.7V(by volume or something. 2.7 is also technical absolute minimums, cutoff voltage is usually more like 3.2V. Please don't abuse the battery in the ranges between 3.2 to 2.7V, let alone below).

Charge/discharge current capacity is constant throughout, at least so battery manufacturers say, at 1-20x the amp-hour capacity depending on the cell. Usually 5x or less.

Since energy = voltage x current, instantaneous W capacity is higher at first, reducing as it becomes supply side limited rather than load side limited.

But all those is irrelevant to why everyone uses mAh, it's because products with biggest numbers sell fastest. Marking capacity in Wh is noble, but it's a clearance worthy sin if you ask the shelves.