I can't deny that it's technically possible. I just think it's extremely far-fetched.
For example, let’s say I buy a Monet painting for X million and a bank is willing to give me a collateralised loan of up to 75% of the asset value. If a bank decides to sell that asset tomorrow (i.e. at a suboptimal time) they might not even get 75% of the value back.
Debt is a lever in business and some people make it big by leveraging big.
It's doubly far-fetched to talk about loans to individuals for $20MM. If you're applying for a $20MM loan, you should have an accountant and your accountant should have told you to put this kind of thing in a corporation. You'll still lose a lot but it shouldn't bankrupt you personally.
But yes, obviously not that common. Random Joe won't be getting these. Random Joe doesn't have $20m in assets either, though.
The most obvious situation is probably where you've invested a certain amount of money in a highly illiquid asset, that can someday (i.e. 10-20 years from now) be liquidated for a significant capital gain. This could, for example, be a business or a plot of valuable land.