Hey, sorry for my late response! Originally there were indeed plans to add a shovel, and actually we even had a shovel in very early versions of the game (before it was released, so the shovel was never publicly available). It's actually a low-poly version of the one from the loading screen

It was intended to be used to dig up soil (while the pickaxe would be used for stone). But it was quite clunky to work with it due to the way how the terrain is smoothed... there are smooth transitions between stone and soil, which made it difficult to have separate items for the terrain materials. At the end of the day, we decided to just have a single tool for digging (the pickaxe) 
There had been considerations over time to use the shovel for different things though, e.g. as a special smoothing tool etc. But I'm a bit afraid that this could be confusing (at first glance, people would think that the shovel would be used for digging soil while the pickaxe would be used for digging stone), so the shovel never made it into the game unfortunately...