The reason for the exemplary unit testing in Ruby (it's not just Ruby... Python and other dynamic languages too) is that these languages do not have a compiler.

They have to unit test everything in order to catch bugs and not all bugs are caught by unit testing hence programs such as pylint try to do things that C++ compilers do every time the code is compiled. You can and should still unit test C++ code to make sure that data are as you expect, etc. A good compiler with all warnings turned on is very helpful.