@Shalvenay Mainly because of EMC. The old school external memories with parallel buses that extend the MCU's address bus are notoriously error prone and weak against EMI. More modern memories with SPI etc still struggle from poor EMC characteristics and you have to mess around with manual CRC. In addition, they bloat the BoM for no good reason. Why have a bad product that cost more when you could have a good product that cost less?
Additionally there's the issues with data retention and limited number or erase cycles, but you have that with on-chip flash as well.
I recently used Arduino in a project revision, because it was already in :s Never again. Was my first time actually working with microcontrollers, but there were way too many things annoying me
@Lundin Oh, I see. You meant the external flash Flash for MCU program storage.
I’m currently designing a board which had an STM32G4, removable microSD card, and an SPI Flash chip SST26VF032B. The SPI Flash is DNP. It would be a backup location for calibration parameters.