As the manager of contractual funds, its essence is the trust relationship established with fund share holders, while fund property is essentially trust property. Trust assets can only be managed and disposed of as the manager's own property, but they do not own the ownership of the trust property, so they cannot be pledged to a third party.
Simply put, only the items you keep cannot be pledged.
As for the argument that the net value fluctuation of contractual funds leads to the inability to pledge, it is even more absurd. Even stocks can be pledged. Why can't relatively stable funds pledge loans? The reason why stocks can be pledged is that shareholders have property ownership, while contract funds do not.