Why are phone batteries quoted in mAh and laptop batteries in Wh?
Amp-hours is the traditional unit for battery capacity, but it's not meaningful for comparing batteries containing different chemistries or different numbers of cells.
Most cars run 12V (nominal) lead-acid batteries, so capacities in amp-hours can be meaningfully compared. Most phones run 3.6V (nominal) lithium-ion cells, so capacities in mAh can be meaningfully compared. Laptops run at various design voltages and cell counts, so comparing their battery capacities has to use a unit that takes both current and voltage into account.
Yes, it would be better if everybody just used watt-hours or (better, to my way of thinking) kilojoules.