first of all, if lenders did the kind of DD you are talking about, it would preclude most ordinary w-2 employees ever getting a mortgage for a home. "they have only one source of income! they could lose it at any time!" these people are a much bigger risk than some landlord trying to get a mortgage for their nth rental property.
to answer your second question: if you, a private individual, bet all your money on investments that go belly up, you're just screwed. if you're a large financial institution, you get to argue that your going under would create rippling consequences that outweigh the erosion of moral hazard that comes with a bailout. it's not fair or reasonable, but it's reality.
