I don't think this is unique to churches, or even non-profits. Plenty of non-church non-profits rely on a few large donors for much or their funding (in fact plenty are designed that way out the gate - they're founded by one very wealthy individual to work on the projects they care about) and plenty of for profit businesses rely on a few large dollar clients for much of their revenue. Both could potentially be seen extensions of the same economic system that concentrates wealth for both individuals and businesses at top.