Same reasoning in the physical world, where McDonald's was always reluctant to locate inside malls (still is). Who wants to be a hostage unless you have no other way of drumming up your own business?
It's another to pay the landlord for every customer that walks in, make heavy renovations and investments to the degree that it's almost impossible to ever move out.
Marc Andreesen has an interesting counterpoint at http://blog.pmarca.com/2007/09/the-three-kinds.html where he holds up Salesforce as an example of a 'Level 3' platform. Of course, his own Ning is another.
