Your argument is perfectly understandable from a superficial accounting point of view. Similar arguments have been made by childless couples and others as to why should they pay for the education of the children of others. No doubt the same argument could be made by the illiterate against the provision of libraries.
Take the analysis one or two steps further and we might see benefits that accrue to all of us regardless of belief, or in the case of the latter examples, to the childless and illiterate. To be really pedantic about it, hospitals were developed by the church in Europe, so a chapel is quite appropriate for a hospital.
Aside from accounting and who owes what to whom, the culture of Australia , for better or worse, happens to be based on the idea Christianity. Most of us are cultural Christians, whether believing the story of Jesus or not.
