Probably the same reason the French don't like the English or the Indians don't like the Pakistanis. The closer two things are to each other, the more passionate people are about the differences.

My experience is that all the languages are identical. Python has a culture of cleanliness at the core, but there is a lot missing from the language that makes it unclean in practices. (See: the investment bank I worked where all new apps were written in a C#/C++/Python trialect, to coin a new word.) Perl is a very messy language, but the community has hyper-overreacted and have made a very clean set of "best practices". I haven't done much Ruby, but I can't say anything other than that it looks like a combination of Perl and Python, with a relatively new community around it.

These three languages are largely the same thing: they run slow and they're fun to write. Hence all the infighting.