A lot of the confusion seems to come from laziness, and no small amount of arrogance that ignorance should be tolerated. Irony that.
As to *why* people have difficulty, I think it comes from trying to match the pattern of gallons, and gallons per time. This is why it is common to see kW/h: the presumption is that gallons are to kW as gallons/h are to kW/h. My wife and I have discussed this more times than I care to count. She understands that one of the units is an amount and the other unit is amount per time ... yet her mind refuses to accept that a rate is not written as something *per time.*
It does no good to write for her
(amount/time)*time, or to explain to her that the watt unit is a ratio by definition. She accepts the algebra as correct but cannot translate the meaning into everyday usage.
She finds her inability to use the units easily VERY frustrating. In her case it is not from lack of effort.
The Australians use the mega-Joule instead of the kWh to denote an energy amount. They lose some calculation convenience but I suspect have less confusion in the units.
2013 LEAF 'S' Model with QC & rear-view camera
Bought off-lease Jan 2017 from N. California
Two years in Colorado, now in NM
03/2018: 58 Ahr, 28k miles
11/2018: 56.16 Ahr, 30k miles
2018 Tesla Model 3 LR, Delivered 6/2018