But how is it that in a world of magic it isn't obviously possible for a few bright-thinking mages to invent the equivalence of kerosene or sterno?Magic and magic users don't make for better scientists. Sure, by the rules, magic-users are highly intelligent, so this isn't a knock on character. The main problem with magic is that it doesn't help advance science, so I don't see how it could have a significant, sustained push on science.
Magic doesn't help with discovery. There are no spells that would allow sight deep into the cosmos compared to or better than a telescope to advance astronomy or its application for ocean bound navigation. Conversely, there are no spells for microscopic vision to advance the fields of biology or medicine. There are no spells to isolate, refine, or process elements and compounds to advance materials science. And so on and so forth.
Magic doesn't help with measurement; scientific inquiry is built on measurement.
Magic doesn't help much with generating, controlling, and applying energy. I'm thinking in the form of engines here. Well, maybe the use of a Permanency and some sort of fire spell could create a fuel-less flame. But magic wouldn't help with the invention of gears, cams, and other power transmission systems to deliver work from the heat generated.
You could look at it the other way too. How would magic have helped classical man devise a method for creating concrete? How would it have helped to create new steel and metal alloys (beyond creating higher temperature forges)?
Magic in D&D is geared towards combat, exploration, and all the fun things that go with the game. It could have just as easily been geared towards science (but what kind of game would that be?). It is not like what I am saying here is any big revelation, but it is both a common sense and convoluted answer to how we can have magic and Medieval/Renaissance technology side by side.
Note that I am making a slight distinction between technology and science. An enterprising magic user can use magic to create some wondrous things, but their magic is not advancing (or creating) science or the scientific method.
Again, I'm not saying that an inventive use of magic couldn't create effects that resemble technological advances, but said effects are not advancing science qua science.