And it would be ridiculous - in a well-designed D&D setting with decent rules. Not my fault 5e gives us neither; so I have my own setting.
It's not like Basic or AD&D did it any better.
Let's face it: much like almost every other fantasy or science fiction franchise out there, D&D has to rely on a "Planet of Hats" for the races because otherwise, it would be complicated and take up too much room.
True; though the alignment is merely a tendency and the different statblock is to reflect actual measurable differences between the species.
But then we are left to wonder why high elves and wood elves, or mountain dwarfs and hill dwarfs, are so biologically different as to give different bonuses, yet humans from the woodlands, the desert, and the tundra are all the same.
Nope. Not even all PCs are literate; except Mages, where it's automatic (otherwise how could they read their spellbooks).
Nope. Most non-adventuring NPCs don't, and not all PCs do either, if the player chooses such or if the character only knows its native language (which can never be Common).
Those are your house rules though--the AD&D books are pretty silent on the subject, unless you use the (optional!) nonweapon proficiencies in 2e. In 3e, everyone was literate save the barbarians (unless they multiclassed, at which point they magically became literate), and in 5e, literacy is assumed for everyone, including barbarians. I don't know 4e's opinion, but I'm guessing it's closer to 3e and 5e than AD&D. Unrealistic? Well... there is
some evidence that medieval people were quite a bit more literate than we had previously thought--I found
another site which suggested that by 1500, half the population could read, although not necessarily write.
(Plus, that's a rather humanocentric view of a setting. Why would elves, dwarfs, and halflings be as illiterate as humans?)
And, of course, this assumes a standard medieval time period. Eberron is higher-tech and has schools--my upcoming Victorian-ish game is like that as well. In a setting where there's gods of writing or knowledge (such as Deneir in the Realms), it's logical to assume that a duty of the clergy might be to go around and teach as many people to read as possible.
Until recently I had a sailor character in my game, and his being a sailor came up constantly while the party was on the coast. A few sessions ago it was relevant whether the party had a leatherworker in its ranks (they didn't) to make little booties for their sled dogs. And so on.
OK? I don't think that having proficiency in leatherworking is a feature in any 5e background--a proficiency, yes, but not a feature.