General question about Freddie, Fannie, and other mortgage lenders...?

I am a little confused as to where this so often talked about loss is coming from in the present economy. I get that x amount of homeowners are defaulting on payments. I get that x amount are being forclosed. I get that x amount are abandoning their mortgage payments, and moving in with momma. I ume a large percent of this mortgage debt for the above mentioned companies are for this reason. What I don't understand is, if the house is still standing and in good shape exactly where does the bulk of the loss come? isn't the overall picture one that more than 95% of homeowners are making their payments? IF the other 5% of homes are just vacated and resold at a later date and logic suggests that eventually new homeowners will come along to buy the foreclosers why such worry? it's not like the homes are being destroyed. Are ysts having us believe all the mortgage notes abandoned total losses? so if you still owe 100,000 on your home and you abandon it the bank is at 100,000 loss?