Because we're often working on a proprietary investment thesis and don't want people knowing what we're doing. During the course of diligence, we may speak to 20+ experts from these networks and don't want people getting a hint of who we are and what we're thinking. That's the premise of these expert networks is you can get advice totally off the grid.
For example, I may call up an expert who is an orthopod or pain doc and ask detailed questions on how exactly they decide which device to use, how reimbursement has trended, how they compare products, post-market results, etc. There's no need for them to know I'm from XYZ firm or working on XYZ thesis. If anything, that may affect their answers. I also don't know these people so I don't want them telling their buddy or the next person they talk to over the expert network that XYZ firm asked them about XYZ device. For all they know I could be an entrepreneur, executive, investor, etc.
Usually they don't seem to care. They spend 45 min telling me what's obvious to them and get paid a few hundred or a thousand bucks and never have to think about the call again.