A couple things to consider: many times an owner will bring a car in for restoration or a lot of work to be done, and leave a "deposit of whatever % of the build. Many times, that is all the money the customer has at the time, thinking, "I'll get the rest of the money later". Time goes by, customer looses interest, or is not as interested as he was in the beginning, shop now has gone thru the deposit in time, and the car just sits. Seen this many times, calling the customer and getting the old "Well, I've been meaning to call, had some unexpected expenses, or the wife found out, I can't spend anymore on the hobby right now, I'll call you when I get more money". That call never comes......
The car now takes up space, other customers stop by, asking why is that car taking so long?
Another common situation, Guy brings a car in for say $9,000.00 worth of work, in a 15 year old pickup truck with an open trailer, with rotted wood floors, and 2 chains holding the car on. Wants to leave $1,000.00 as a deposit. Do you ask him if he will have the balance due when the car is ready in 30 days? It's a touchy situation, have seen it often, especially with drag racers.
It's not always the shop is dragging their feet.....