Most people want an estimate because their state has estimate laws like Utah where I come from. These laws say that the estimate must come within a certain amount of the actual work on the car. First I will explain why a collision shop can estimate the work on your car.
I used to own a collision repair shop. They have books and computer programs with lists of all parts and labor necessary to do the job. These aren't right on the money but they are close enough to do a great job of the estate on the collision repair.
The process of a restoration is completely different, there is no standard for times that is takes to repair rust or locate hard to find parts because it is impossible to estimate these things. To properly estimate a restoration project the shop would no doubt have to tear the car down.
Because they would need to inspect every part of the car to thoroughly estimate the restoration of your car. By the time a shop tares down your car the project is already started. They have to charge you for this and the reassembly process before you pick up the car.
It's pretty much a waste of time for the shop if you decide you don't want to restore your car. If you don't want to pay for tare down and reassembly the only recourse that the shop has is to lean your car so they can sell it and get paid for the work.
There is no way for the shop to tell what's going on underneath your car, and with wiring, mechanical and electrical just by looking at the outside of the car. They have to get it and look for rust, burned wiring, bad brake system parts, and inferior workings.
When a shop tells you that they cannot estimate a restoration project it is because they can't. There are no databases, books or any of the computer programs that a collision repair shop has. Personally I won't even give a ball park because most people think that it's the same.
A restoration shop will lose money on every job that it estimates unless it's just light mechanical or body work. Such as a tune up, oil change, brake shoe and pad replacement, these types of things can be estimated before the work is done.
To sum it up: Plan to spend a lot of money to restore your classic car, when the shop tells you that they can't estimate the work it's because they can't. The best thing that you can do is to have a full on plan to restore your car.
Most shops will help with this if they are going to get the work in their shop. I know that I am always glad to help a customer with a comprehensive plan to restore their car. It is still not an estimate but you know what's coming during the process.
It will prepare you for what is about to come during the restoration of your car. Most shops look to build lifetime partnerships with their customers, and will do what it take to make this happen. It's just good business to make them happy.