Technology aside, here's why you don't ask why D&D world with magic keeps the broad feel of medieval society. Magic would remake society in ways we never anticipate--not into a magic-powered 1921 like Eberron, but into something as unimaginable as 2021 is from 1221.
Just 1 low-level spell would remake medieval society. For example,
Lesser Restoration
Zone of Truth
Purify Food and Drink
Sending
Spells like Alter Self, Charm Person, Distort Value, and Suggestion would completely change norms of behavior.
Lesser Restoration
Zone of Truth
Purify Food and Drink
Sending
Spells like Alter Self, Charm Person, Distort Value, and Suggestion would completely change norms of behavior.
Now multiply that upheaval by the D&D spell list. You don't even have to imagine what happens when folks start inventing useful spells for *non-adventurers*, which would totally happen.
If you enjoy the entertaining fiction of a typical D&D world, then avoid pulling those threads.
If you fancy the mental exercise of world building, have at it, but you'll lose the resonance of D&D worlds gain by sticking to familiar elements of myth and history.
If you fancy the mental exercise of world building, have at it, but you'll lose the resonance of D&D worlds gain by sticking to familiar elements of myth and history.