It's an attempt to get the most "bang for the buck". Essentially you write lots of small programs which have limited and well defined functionality, then you hook them up any way you like. In fact taken to the extreme (as with Plan9) you can do anything with simple shell scripts.
BTW there are simpler developing environments out there which have a decent feature set, without the complexity of a C(++) toolchain. Lazarus is just one example of it. Of course you then loose flexibility. Lazarus, for example, is mostly suitable for GUI applications. Writing a webserver with it is hard. Of course it does GUI decently well, allowing you to have one codebase compiling from everything from your bog standard Linxux (GTK) over MacOSX, Android to even exotic platforms like Win32.