If you have a stamping plant, you usually have far fewer presses than part numbers. You stamp out some part, then change dies to stamp out another part. Die changing and adjustment is time-consuming and is non-productive downtime. Once a press and die set is running well, it will produce parts at a good rate without much attention, but die changing is labor-intensive. The less often you change, the lower the cost per unit.
The optimal batch size at the stamping plant may be quite different from the batch size the assembly plant wants. That means in-process inventory, stored somewhere.
In the scale of auto body parts, dies weigh many tons and cranes are needed to do a die change. Clamping a die into position is complicated. There are quick-change systems. They're expensive.[1][2]
What's probably choking Tesla is that they planned their stamping batch sizes for a much higher production rate than they can currently achieve. Much of the planning for the Model 3 production line was based on the assumption that not much would go wrong. They made hard tooling for parts before using prototype soft tooling to save time.[3] That's a win only if you get it right the first time.
(I once worked in an R&D facility for heavy hydraulic equipment. They had to deal with problems like this. People talked about "setup" a lot. A basic fact of manufacturing is that setup and changes are much more difficult than production. Once you get a production machine set up, it's mostly putting in blanks and taking out product, and that's often automatic. Setup requires skilled people. There's been a lot of talk about "agile production" and a "lot size of one", but the real economies still appear only when you make large numbers of something. Look at the pricing for some on-line service that makes custom things for you. There will be a big price break as the quantity goes up.)
[1] http://www.thefabricator.com/article/stamping/10-common-quic...
[2] http://www.thefabricator.com/article/stamping/how-quick-die-...
[3] https://insideevs.com/tesla-insider-speaks-out-on-problems-o...