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...