They do not have to be. For instance, it is common to never completely pay off the mortgage in Switzerland to avoid one-off taxation. Instead, the eventual taxes are included in the cost of the debt.
An interest only loan doesn't suddenly mean there's no repayments. The hint is in the name, you still have to repeat the interest on the loan. Payment terms may vary but if you think that banks across the US are sitting on interest only, asset backed mortgages for the ultra wealthy's most expensive purchases, and taking no payment from them then I don't think we're going to be able to come to an agreement on this topic.
