What other asset-backed-security is tied to someone's life until they die or become physically incapacitated?
The concern is that too many would immediately declare bankruptcy after finishing their education, wait out the 7? years it takes to for that to fall off their credit report and then get on with their lives. Since few new graduates need serious credit for anything beyond cars....
Therefore securities built on top of these loans, such as the pools you refer to in your link, are by definition not "asset-backed-securit[ies]". They are instead backed by e.g. the laws and regulations that make them non-dischargeable ... and I contend that's a major reason why they are non-dischargeable.
The student loans themselves are assets (the fit the above definition), my point is that they themselves are not asset based like a house or car loan.
Sallie Mae's use of the term "asset" here might be slightly misleading, but being essentially part of the government they can get away with it.