Dark Skinned High Elves Do exist in Lore, btw.
https://wow.gamepedia.com/Devi
She's from an alternative timeline, but her dark skin color is not remarked as something exceptional, or odd. Not even a hint of a mixed parentage, she's just presented as a dark skinned elf.
But I dunno man, of all the hills to die on "there are no dark skinned elves" is just such a worthless one , specially when it's just me drawing my character, and it's like I'm personally offending you with the "inaccuracy" and "inconsistency"
Regardless of such, I'd like to see expanded skin tones for every race, and in terms of retcons it's far lower than much more egregious changes, specially when skin tone doesn't have to carry any sociocultural weight in-group in azeroth, thus existing as an aesthetic choice. If all the answer you have to the idea of more expansive skin tone selection is "is inaccurate to the lore" then you both lack imagination and are sweeping under the carpet all the far more drastic changes in lore we have already
But again, weird hill to die on.
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Not necessarily. The inuit people live on the artic and have dark skin; several groups of non european lineage that live far south or north have dark skin tones, so it's less of an evolutionary rule (fair skin to get vitamin d) and more of a possibility.
And specifically for the World of Warcraft, it really comes down to how sunlight could have affected the diverse range of Kaldorei skin, specially kaldorei living on different climates given how expansive the Kaldorei empire was.
When it comes to it, the reasons why high elves are fair skinned aren't more deeper than Warcraft taking their cues from mainstream Lotro inspired Fantasy.
Knowing the Blizz devs, wouldn't surprise me that if we ever got dark skinned elves it would be from an ecuatorian island lodge previously undisturbed, so who knows heh.