Because, unlike the chainsaw example you provided, one can simply give back the chainsaw if the rent becomes too much of a burden. Can't really do that with (fiat) money because oftentimes it was borrowed to pay off / purchase something else; something which the lender could care less to get in return. Today you have people in (literal) slavery because of borrowing and interest (and sometimes interest on interest).
Recall also that with modern fiat currency based upon fractional reserve banking, money kept in a savings account is losing value due to inflation. In the past 10 - 15 years, I do not think that there has been a savings account or CD which has kept ahead of inflation. In his book Barren Metals, E. Michael Jones makes a good case that the interest profited from said accounts isn't usury because it is still losing value over the long run.
You make a good point here. However, in the case of having borrowed money to buy a car or a home, if you go delinquent on the loan, the lender can always just take the car or the home back. Many other types of loans require collateral, so that if the borrower can't repay the loan, they forfeit the collateral.
Unsecured loans, or loans for which there is no security or collateral at issue --- how do you give back a college education, or for that matter, something that ceases to be once you've used it (a vacation, or even food)? --- yep, that's a problem.
Best not to go into debt for things where you can't either give back the secured property or the collateral. Now that
is debt slavery.
But why is it, that the Church once taught that any usury whatsoever is sinful, whereas now, usury rising to the level of sinfulness (i.e., exorbitant interest) only exists when the obligation exceeds what is commonly perceived as just? Again, talk to me like I'm five.
The only thing that remotely makes sense to me, is that in the era prior to the existence of fiat money, there were only so many ducats, or shillings, or what have you, in existence, and you are having to pay with something that you don't have, something about money once not having been fecund, and in modern times, it
is fecund. Money no longer has intrinsic value, but rather, it is created by fiat, supposedly to reflect the value of labor, goods, and services in the economy, and to facilitate trading and store the value of these tangibles and intangibles --- in other words, if I have a dozen eggs to sell for a dollar, I don't have to go out and seek someone who has something I want, that is worth a dollar. The dollar acts as a kind of "credit" to help the two transactions "slide past one another" without necessitating barter.
I supposedly have a degree in this kind of thing, but again, I still just don't get it either way.