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 have the ownership of the trust property, so they cannot be pledged to a third party.
Simply put, an item that only you keep cannot be pledged.
As for the fluctuation of the net value of contractual funds, it is ridiculous that they cannot be pledged. Even stocks can be pledged. Why can't relatively stable funds pledge loans? The reason why stocks can be pledged is because shareholders have property ownership, but contract funds do not.