Because the bank would never let you borrow a billion dollars unless you could prove to them that you also have the income AND the discipline to service such a loan. For that you'll have to show them quite a bit about your personal economy, which may even include showing them your tax bill. They'll also want papers that prove that they have a stake (mortgage) in what you're loaning money for. On the other hand, if you know your way around law and certain organizational structures, then there are more ways than one to get quite exorbitant loans depending how you go about it. ;)