And so what?
Of course people that contribute for fun will prefer re-writing to fixing bugs (*). Then it is expected that people making a profit of it - from simply using it to packaging and distributing it - will do the most boring parts.
It is a system far from perfect, but the important thing is that in many (most?) cases IT WORKS. The proof is that many people make use of open source software with no more troubles than using close source software owned by some company.
(*) Although as a developer (not of OSS) I think that hunting and fixing bugs _is_ fun. The non-fun part is dealing with people that believes a particular behavior is a bug only because they don't like it.