Where there are assets there must be liabilities. WAMU held $300B+ in assets but also $270B+ in liabilities, therefore making those assets not worth anything close to $300B once liabilities are paid.
For example, if the underlying loans of MBS's were funded by bank deposits, those deposit liabilities must be repaid. The actual value of the asset comes from the interest payments only, not the total asset.
If the loans were funded using company profits then the opposite would apply since both the principle and interest payments would be credited to the company. Here $30B in loan assets have an actual value of ~$30B since there are no funding liabilities.