Shovels get more basic mineral resources in larger voxel areas, they should be used for digging out areas, or if you need special minerals (like sulfur). Pickaxes do smaller voxel areas and get more of the valuable scannable ore resources and few of the special minerals. That assumes they balanced each tech tier consistently so it makes sense, higher tier get so much it can be hard to track since the UI log severly lags behind your actual actions.
Mining is already so fast that you clear out a voxel within a few hits even with wood tools. It is very easy to mess up your voxel cube outline placement because of the animation time lag between control and result, people have always complained they cannot dig smooth areas because of this, they will always overstrike and leave undesired holes (This is still the case even after much iteration the mining UI in alpha/beta)
Adding a follower hitting your same voxel would just make that problem worse as you would easily get out of sync on who gets the last wack, and mining is already hard enough to get a workable camera view without your follower standing in your voxel way. Having them dig their own voxels would lead to disastrous results because they do not have your intelligence and mining plan (although there was requests for robots that could follow a 3D plan drawn using build mode tools there is always a concern that is unattended macroing that unbalances multiplayer)
If you just want more resources tell your followers to work while you are mining, they will go off and get a container of mined resources, then you tell them to get to work again when they come back. This way you get more resources, they are doing something, while you still maintain voxel control over your mine layout, and not have a chaotic mess that you and your followers cannot get out of. But that container should be constrained to contain only the metals and minerals that are in the surrounding voxel area, so that if you are working gold your followers also get gold.
Trees are different because they are isolated meshes, there is no worry of overstriking a tree and taking out the neighboring tree. That is why followers will help you axe a tree, but not pickaxe a voxel.