Didn't read OP in the interest of answering the thread title. I'll edit if necessary.
Why? I'm not very good with money. Even with a budget, I manage to have something come up I need to pay. Petrol, phone bill, broken video card, so on. Preorder in allows me to say "well that's gone, can't spend it."
That being said, I don't preorder often. Only if I know I'll be getting a game regardless, like Fallout 4 and Halo: MCC (bad example, but I still would have bought it eventually).
Basically, if it's a day one purchase, it's often a pre order. Also, EB allow you to return games for full price up to 7 days. Obviously I can't do that with Fallout 4 (never getting a Bethesda game on console again), but most games I buy, I can.