They don't have the necessary time horizon of multiple years. Some of them are out of business, others are scrambling for liquidity to survive.

As for the assumptions, you can use whatever assumptions you consider reasonable. I grant that, if you assume 90% of existing subprime mortgages go to foreclosure with 90% loss, then this class of mortgages is essentially valueless, even the senior tranches. It is up to each person to decide how likely that seems.