Having a part number in my system does not mean that it is a reasonable case for a spare part.
These are actual examples from my business: I have part numbers for raw powders that I have not yet sintered into ceramics. I have part numbers for parts that go into a welded and inseparable sub-assemblies. I have part numbers for components that are bonded together with glass seals that cannot be repaired if separated.
It is easy to see how you cannot just use the BOM to determine the list of spare parts and it is easy to see how a manufacturer could simply pot their whole unit in epoxy for "structural" reasons and prevent any individual spare part or sub-assembly from being useful.