I think the main appeal is to work with small devices that have well defined resources. Just consider the popularity of the ATtiny85.

However, low end 32bit devices are also available from reputed suppliers. In fact, STM just announced the STM32C* series which is also aimed at replacing 8 bit controllers.

The cost benefit may only be true for low numbers and when sourcing from distributors. I am fairly certain that the price advantage is much less when talking about actual production volumes

*https://www.st.com/en/microcontrollers-microprocessors/stm32...