>>>Why hasn't Middle Earth had an Industrial Revolution?

Because they studied physics and understood that the industrial revolution leads to doom.

The exponential GDP growth is us considering natural resources free and unlimited in our economic model, while there is in fact a finite amount of resource (we've passed peak production crude oil in 2018). This very free use of resource leads to climate change, increasing the pressure on the production of 'renewable' resource like food. In physics, no infinite growth in a finite planet.

Their world sustained civilization for 45,000 years because they didn't have an industrial revolution. Our civilization is ~11,000 years old at best, I'm not entirely convinced the GDP curve will continue to grow exponentially for the next 34,000 years.