In automotive electronics ECUs, why is an external EPROM IC often used instead of the MCU's internal EPROM?
I used to work in automotive, and there could be any number of reasons to speculate on. First and most obvious is size -- if the internal EEPROM (or other non-volatile media) isn't large enough, an external part is obviously needed.

Next is qualification. If a SW team has already written a driver for an EEPROM and HW has qualified that it meets AEC-Q100 and other internal tests, it might be "cheaper" to use the external device initially to meet schedule, and revise the design at a later date to optimize for cost.

Additionally, in the old days, the EEPROM was a socketed device that was programmed externally and then placed on the boards during assembly. Often you'll see aftermarket modifications that change the contents of that device; modern tuning companies just reflash the MCU itself, though.