Because it's not a real church and IMHO, the donations are not true donations, either. Scientology is a business.
IMHO, The big donors (the "whales") are getting something back for it. Something financial. Did you see Tony Ortega's latest blog?
http://tonyortega.org/
Real estate involves a lot of money - commissions to realtors, companies hired for renovations, law firms hired to write contracts - just for starters. COS also pays a 10% "Field Staff Member" (FSM) commission to those that sign up new members. That's 10% on all their services - forever. Unless someone at COS approves paying a different FSM.
I believe most of these whales are actually "investors". They are investing in a network that will send them money in the way of contracts or some other repayment.
Even for scn, this is far too much money for them not to be getting some sort of kickbacks. That's my view, anyway.