Why are there no nonexponential nonlinear loans?

There are non-exponential loans. What is considered the norm in the West would be considered as "usury" in many countries of the world. Especially when interest rates of 20-30% are charged in some cases.

https://en.wikipedia.org/wiki/Usury

Even in today's world, there are loans with extremely low or even nil interest rates. Some institutions have a 'Rent to Own' setup whereby you have rental payments over the course of the loan so that at the end of the loan-period you own the asset.

https://ijarahfinance.com.au/islamic-banks/