The money spent paying someone to set up a Mac server somewhere else, getting that somewhere else connected to your AWS networks, and ensuring that connection is secure, and generally integrating it into your existing cloud devops infrastructure (or, perhaps more expensively, not integrating it into your existing devops infrastructure), would, I'm guessing, be horrendously expensive compared to the premium Amazon charges on a relatively turnkey solution for use cases such as, "We need to run the occasional Darwin build."