Why does technology not evolve in D&D?

A central conceit of D&D fantasy worlds is that you are living in the shadow of a bygone era. Sometime in the ancient past was a golden age of magic and technology, of which we can only scavenge their remnants in their ruins and tombs.

Though there are monsters and dragons and secret ultra-conservative societies to blame in our setting that prevent the Agricultural Revolution that also has to happen for an Industrial Revolution to take place, but a more fundamental reason is that in that bygone era, they consumed the resources necessary to spark an industrial revolution. If our own society collapsed and were regressed into the stone age, there will never be another industrial revolution (at least as we know it), because we have mined out all the materials necessary for that to happen. All the easy sources of oil, coal, and minerals have been tapped and it would take millions of years to replenish them.

“The next main reason for the start of the Industrial Revolution was the mining of resources such as coal, which were vital to the processes of industrialization. For instance, Britain was the first to industrialize and had large supplies of coal present in the country. Coal was a necessary ingredient in the industrial process as it fueled the steam engines that were used in trains, ships and all other sorts of machinery. Not only did Britain have large supplies of the resource, but it was also easily obtainable. Unlike other European nations, coal in Britain was relatively close to the surface and was therefore relatively easy for miners to find and extract it. The mining of coal became even easier after the invention of the steam engine by Thomas Newcomen, which was originally used to pump water out of coal mines.” - (https://www.historycrunch.com/causes-of-the-industrial-revolution.html#/)

Back in the Age of Legends, it wasn’t just a utopia of incredible magic, but also incredible industry. Since they consumed all the easy sources of coal, oil, steel, etc, you would need technology from the Age of Legends (or comparable) to be able to mine it in the quantities that they did. And without an Agricultural Revolution to take people off their farms, you never get the manpower and brain power you need for industrialization.

This is why societies never quite get past Steampunk & Magitech levels of technology - they don’t have the concentrated sources of fuel that you need to get it going.