Qitzur Shulchan Arukh – 179:12
Just as one has to be careful in guarding something you were charged with, all the more so one must be careful in guarding collateral, because he is like a paid guardian [one of the four categories of guardian discussed in the gemara] for the collateral. Just as someone appointed [to guard] can not give someone else the item for them to guard (as will be explained in chapter 188) so too the lender can not give the collateral to another’s domain to appoint them [as guardian] or use it as [his own collateral] without the knowledge of the owners.
The lender is like a paid guard, whether because he gets a benefit by holding onto the collateral; it mitigates his risk of non-payment or because the loan is a mitzvah, and thus the guarding is being “paid for” with that reward. See Bava Metzi’ah 81b-82b.
We discussed numerous laws against obtaining benefit from a borrower because of a loan, since it would be a form of non-fiscal interest. And yet, even though this risk mitigation is here given implied monetary value, this reasoning does not prohibit receiving collateral. I do not have an explanation.