Because you can either do it with excel spreadsheets or insanely expensive PDM/ERP software like SAP which is insanely expensive to setup and anyone who has ever used it will tell you is terrible. There used to be middle road options like netsuite but they were bought by SAP. There are alsoneays to do it in Catia, etc like the airplane folks do, but that's also insanely expensive.

People are now using airtable to do it

Mapping physical actions (parts consumption and receiving) to digital transactions requires both tech and human behavior. Usually the behavior is the bottleneck because it's a lot easier to just take a part off the shelf than it is to create a work order, define a BoM, and post a transaction to the ledgers. At places like this -- the craftsman/engineers are usually the guys with authority -- not the process guys. So they grab a part off the shelf, and try to play catch-up in Excel later.

To add to the other replies here.

From my perspective and experience, the actual reality of manufacturing anything complex does not match up with the set of rules that has to constrain any given ERP platform to be a useful, data-based record. Allow too much freeform data, and start losing more and more analysis ability. Too strict, and you quickly crash against the fuzziness of reality.

All the while requiring that your data be as close to 100% accurate as possible. That's the crux of the problem. Modern, complex engineering/manufacturing requires a level of traceability that cannot be handwaved away.

Now contend with this problem against a bill of materials tens of thousands of lines long with nearly a million discrete items put in, all with their necessary interdependecies recorded.

Now do this across an entire product line.

https://michelbaudin.com/2018/08/01/the-bom-rap/#more-126364...

^ A good read on the complexity of BOMs and why managing them can be tricky.