Probably for the same reason that noone makes a rival to:
http://sourceforge.net/projects/funambol/ and other AGPL software which requires code signover to get a commit bit..
that spinning up a rival project with insufficient resources based on a fork is cost / time / resource and mind-share prohibitive..
but hey, funambol is 'free software' right.. just try to make your own competing cloud storage company based on their 'open code' -
you cant.. so they have an effective monopoly of this type of application commercially, yet while maintaining strict 'GNU' 'freedom'
standards..
software doesn't exist in a vaccum - although if you're paycheck has come from a university your entire life and you've never had
to work to earn a dime for yourself you might think otherwise and thusly pontificate.. (cough RMS)
plus, in the GPL case you mention, this is a n00b jerk move and everyone will think you're a weenie, and they would be correct.