Once you adventure into space and get a little bit of niobium and start harvesting tungsten, thermium is much more available than steel. Collecting the output from a couple of volcanoes is much easier than getting lime.
Why use steel once you have access to thermium?
I have over 3000 hours in the game and I never realized this. I just loaded in my 1265 cycle old map and I saw two shove voles towards the bottom of my map - well below my edge-to-edge break. I thought I eliminated all the wild ones. They are actually dangerously close to a large vacuumed out area that would overheat like crazy if something were to die and rot in there.