The reason for this is that they are typically creating a model for "manufacture".
These models using programs such as "Inventor" are a great way to design and develop equipment for manufacture but don't reuse that model for use as your BIM model to share with consultants... it's way too big!
Autodesk is currently previewing new software that will "Shrink-wrap" and simplify an Inventor model and also removes Intellectual Property.
It's currently a Technology preview and I hope it takes off and is adopted by mechanical equipment manufacturers as part of their process when releasing a BIM model.
Check it out....