You basically have three major hold backs:

* Parts requiring manual assembly. My first summer job was at an auto supplier. It involved running 3 wires though a piece of conduit and wrapping it with electrical tape. It was a connector for some sedan brake light. Family friend. They still run the business the same way. Parts change too frequently and are too hard to automate.

* automation still requires maintenance. This isn’t just replacing parts on machines. This is also the entire supply of parts that go into fixing machines. Lines going down is incredibly expensive. Parts aren’t always readily available. There’s a whole industry of small “tool and die” shops that have the capability to build a part from specs next day.

* plants are huge and expensive., you don’t retool unless you have good justification. Typically, this happens with a model refresh.