To contextualize this, in my experience this doesn't happen as a result of some malicious conspiracy but rather as a natural outcome of working with various projects and exchanging with colleagues in various projects.

For the consultancy it boils down to not reinventing the wheel, learning from others and sharing knowledge about what works and what doesn't. Typically, what is shared are anonymized generic design or cases and not actual code or design files though.

Of course what is knowledge sharing and efficiency for consultants might simply boil down to IP theft for the affected companies. My question is rather how much of it is natural human social interaction and collaboration if you put dozens of people from dozens of projects into one room talking about a similar problem.