In short, by borrowing, the tax code assumes that long term the proceeds of the loan will be disposed of in an appropriate tax way. However there are so many ways to be slippery about repayment that it’s hardly a guarantee. So it may be wise to shift to pessimistically assuming long term shenanigans at borrowing time and taxing the proceeds as income, with tax breaks around “sane” repayment to handle the intended “avoid double taxation” behavior.