How do we even know that its rare?
Oh, thats right. We know everything there is to know about the universe.
They posit it as rare because they can’t even prove neutron stars - a physical absurdity - exist. This kind of hypothesis is what you get when you have grant-driven institutional science unaccountable for the divergence of its conclusions with observational evidence.
Thus, it is dogma that physical absurdities like black holes, neutron stars, dark matter/dark energy, and pulsars exist, because government money is spent to study them and no government money is spent on alternative theories that may better explain the phenomena we see.
Here’s an interesting discussion of neutron stars which may prove enlightening: http://www.youtube.com/watch?v=tiPmoFmBnN8