Because this is not considered as an issue. The message is clear enough and as you said there is solution of cleanup which you know without searching. In such case if people know something without searching they don’t create any issue and just your question is not common. 
Keep in mind that on the forum many users reading your post have no idea how you think like they have no idea why recompiling all dependencies is not desired for you. As you said recompiling all dependencies is of course good idea and people do it by default in almost all cases - at least those I am aware of. However if for some reason you don’t want to compile all dependencies then you can simply recompile just one. 
mix deps.compile dep_name --force
You need to improve your documentation research skills. What’s may be interesting is mix help task which lists all tasks available and mix help task.name which shows the documentation for said task. I believe that deps.compile is obvious naming, so there is no need to describe what it does. Finally the documentation in mix help deps.compile should be enough clear for you. 
Personally I recommend reading documentation a lot. Of course not to remember everything, but to more or less know where you can find some information and also you may find lots of interesting things that would be helpful for you now or in near future and you were just was not aware of their existence. The common mistake is to not read Erlang documentation assuming that Elixir core is all what you need. For example I have found :erlang.term_to_binary/1 and :erlang.binary_to_term/1 very useful. This is pretty basic and simple to understand thing new developers have completely no idea about. 
Edit: As @kokolegorille said firstly you need to fix the problem with your Elixir installation. After this you can use mix deps.compile to recompile only those dependencies you want.
