When you create the SPV in advance, it's very clear what part of the work done by the organization attaches to it (because the organization ensures that all its processes explicitly specify the legal compartment they're running under.)
When you create an SPV after-the-fact, you have to go back and reverse-engineer a separation of liabilities from documents that don't specify whether they're work done for the organization or the SPV (because the SPV didn't exist.)
It's like a divorce. (Or, for an even more on-the-nose analogy, it's like trying to use a condom after-the-fact by extracting any bodily contamination and putting it in the condom.)