It's semantics because they are their own type of financial vehicle. They are a security that is registered with the SEC. They give the buyer an equitable stake at the instrument(property).
"Equity (finance), the value of an ownership interest in property"
Financial Language 101
So are you going to buy some of the mortgage backed from the secondary market I showed you ?
I can call them equities and you can call them bonds but in the end they are not worth what they were supposed to be worth. And worse,what happens if the IRS goes back and takes away tax exempt status because most of the trusts did not securitize the deeds properly ? Does that tax money get taken from the trust and the owners of the mortgage backed get screwed again ? And if the Feds don't do anything because it is their money being used to pay off deficiencies,that would make IRS rules for REMICs redundant. Would you call that a loopole? Or an ambiguouscheat/escheat ? Which is what this mortgage system is all about.
"In some jurisdictions, escheat can also occur when an entity, typically a bank, credit union or other financial institution, holds money or property, and the property goes unclaimed, for instance, by lack of deposits, withdrawals or any other transactions for a lengthy time in a liquid account. In many jurisdictions, if the owner cannot be located, such property can be revocably escheated to the government."
News 
Market Data 
Discover 
