This statement here is the key. This is why many times real estate investments work. You more likely to pay monthly payments on a loan with discipline, than have discipline to DCA into a index.
People think and work very differently under 'compulsions' and 'freedom'. There is always a phone to buy, car to change, vacation to take, and eating out to do with the money if you were not forced to put it away, like a compulsory monthly payment.
Even in cases where I have seen disciplined folks, one emergency/pleasure life situation is enough to ensure they fall of the wagon(spend thrift'ism).
This is why most calculations that don't take into account emotional aspects of investments are doomed to fail. And they almost always do. One thing nearly everyone is bad at is waiting, with patience.