Why do societies progress at different rates?
Tangent, but there's a book on this topic, which rather impressed me: "Guns, Germs and Steel".
So, we have the real-world range of historical developments, as reference. It's a wide range.
And then there's magic. What can we say about the development of magical item production, that's rooted in real-world history? How closely shared is your understanding and my understanding, of how one researches magic, of how long ago was the FIRST EVER casting of Fireball? Has Fireball been written in spell books, in the setting, longer than, say, armorers have been forging plate mail? Or vice versa?
Even though we can take the D&D spell lists as canonical, those questions are setting questions, and every D&D setting is substantially divergent from anything in real-world history.
Though that DOES give me an idea for a setting. Let's say, take real world history, and then around 1066 AD, the Earth's magnetic field reverses, contact with other planes becomes possible, and D&D-style magic becomes possible. The appearance of extraplanar creatures causes religious upheaval. People stumble across spells. The first-ever casting of Fireball is a disaster, but the caster survives and teaches the spell formula to other mages. Druids appear; how closely they match their predecessors of Julius Caesar's time is arguable, but the power of their magic is not. As in Shadowrun, some humans give birth to children who are elves, dwarves, gnomes or halflings; some families are horrified, others consider their pointy-eared or hairy-footed children a miraculous gift.
The PCs are born in, say, 1101 AD. They grew up in a time of transition. Their grandparents remember the days when no one ever saw a spell cast, and when no one had ever seen an elf, dwarf or halfling. If one of them becomes a 9th level wizard... they won't be able to copy anyone's list of 5th level spells, because *no one before them has ever reached 9th level*; they become the INVENTOR of the world's first-ever 5th level spell formulas. (Though who knows what's being invented on other continents - well, until the Aztecs fly across the Atlantic, and demonstrate their set of spells at the Battle of Gibraltar in 1123, and then western Europeans become very darn aware of what's been invented in the Americas, such as the Hunger of Hadar.)
The oldest living elf was born in 1072, and hasn't yet reached full adulthood. The first dwarf born to dwarven parents, rather than to human parents, was born *last year*; there are, as yet, no cities founded by elves, dwarves or halflings, though there's already an Elven Quarter in some cities, and some mining towns have become all-dwarf enclaves.
Any +1 sword or other magic item has, necessarily, been forged, or at least enchanted, within the past 60 years.
There are, so far, a total of six cloaks of elvenkind in Europe, and four in North Africa, but with rumors of many more arriving by the spice trades route from India.
"NOW how much would you pay?"