Spending habits are extraordinarily social, though. "Keeping up with the Joneses" is, by all accounts, how humans actually think and act, in terms of competitive goods(e.g., houses in good school districts) status goods(e.g., driving a Cadillac), and personal consumption goods(e.g., trying to have as much fun on your vacation as your friends did on theirs).

If you focus competition on only personal spending, and let charity wither away, then even if everyone is actually charitable(and data says that the vast majority are), most won't donate significant sums, because they're too committed to spending competitively on consumption.

Also, LW is a pretty in-groupy sort of place. I have no objection to starting small. I think it could in principle be expanded - the fact that charity was all given to the same recipient in church tithes does not seem essential to the process - but even if we make it a social norm among us and try to expand it to our friends, we'd be doing good work.