Not sure there were any strict guidelines back then so it could be whatever the investing strategy of the company was or how good the supporting assets were. It was done to give confidence to 3rd party investors by showing them that the issuing company also had interests in the investment, thus giving it more credibility (improved credit rating).
Look just to name a few there are many more. Now if what you say is true that they were sold off. I mean if I bought those trusts I would of named them My Own Personal Trust fund not kept it as WAMU trust which as you can see was the depositor in all of them. Common all of these trusts have been earning interests since around 2007... Trustee DST due November 2036 https://www.sec.gov/Archives/edgar/data/1317069/000095011706004036/a42800.htm
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