FLOSS developers, particularly those writing code in their spare time, hack on whatever they feel like. Usually because they find it fun.
This is why we have jokes about the number of people writing IRC clients in the 90s, which apply to the number of people writing Twitter clients this decade. :-)
GNOME is a meritocratic collective of individuals (some of whom work for companies with an interest in GNOME) with a degree of aligned vision and values, and an incredibly thin layer of artificial technical leadership (mainly represented by the Release Team).
It's not a command-and-control culture. People work on things because they want to, are paid to, or (in many cases) both.
One thing to keep in mind: There is a sense of GNOME being a "team" of like-minded souls, so distinctions between vision and values between projects (such as Firefox and Pidgin) may stand out to developers more than they are obvious to users.
What one developer finds fun may be another hacker's dental surgery nightmare. There are always new ideas to muck around with, new projects to contribute to, and new people to work with. For these reasons, directing FLOSS developer energy is not a zero-sum game.
When someone says "GNOME and KDE developers should combine their talents and work together, then they'd all be better off (as will users)", my response is this: Are you HIGH? These people may share some common values, but in large part, their vision and values are different enough that it would not make any sense at all, let alone be fun.
Should all the NoSQL folk stop working on competing stacks, and work together for the "betterment" of themselves and their users? Hell no.
Give it a few years and there'll be obvious frontrunners which have been better maintained, supported, managed and better suit real requirements (cf. MySQL and PostgreSQL in the FLOSS RDBMS world), but for now, get the fuck out of the way of innovation!
There are long and interesting stories behind the specific projects the author raises -- the Epiphany browser and the Empathy IM client -- but they're a bit long for this post.
I'm suspect the author would be surprised to find out that the existence of Epiphany and Empathy is much more to do with a sense of exceptionalism on the part of Firefox and Pidgin than of GNOME. :-)
Reality check: Most GNOME users still use Firefox and Pidgin anyway (at least until Ubuntu shipped Empathy in Lucid, and which will be much more convincing in Maverick).