Money has noting to do with it, in the grand scheme of things it is cheap money for the school. Applications aren't a money making thing for the schools.
The real reason is to appear more selective. If 1,500 people apply to HBS and 1,000 are admitted, it doesnt look as good as accepting 8% to people who look at schools. Selectivity is factored into some rankings, and it gives students/alums the feeling of being one of the elite. In reality there are a small percentage of students who are a surefire admit to wherever they apply, then a lot that get into some schools they apply and not others. There is so much overlap at schools with top applicants, very few people every pull a clean sweep.
A few years ago we looked at applicants on here for top programs and found the vast majority got at least one admit to one of the top schools. In the end you never really know unless you try. That said best to be realistic. If you are borderline candidate for Duke you probably dont want to only apply to Stanford, HBS, and Wharton.