Because they would need to inspect every part of the car to thoroughly estimate the restoration of your car. By the time a shop tears 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 tear down and reassembly the only recourse that the shop has is to learn 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 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 takes to make this happen. It's just good business to make them happy.