Said software resides in ROM - Read-Only Memory - and cannot be modified unless the chip is extracted from the phone and installed in a special EEPROM reader that permits/has the capability to write to the chip. The phone’s hardware literally lacks the electrical connections needed to write to that chip. Plus, these chips nearly always had so little extra room (as a cost-saving measure) that something as simple as saved phone numbers needed to be offloaded to the SIM card’s storage - from which it was impossible to run additional apps because, again, that functionality just didn’t exist.

And in many of these older feature phones, the in-memory footprint of actively running software was also well known, so RAM was limited (again, to save on costs) to handle only known apps, thereby leaving vanishingly little headroom and making an in-RAM exploit to piggyback in extra software equally difficult. Finally, most true feature phones didn’t have data connections, making it almost impossible to introduce data into memory.

About the only way data could get in would have been via SMS, and a feature phone has none of the SMS capabilities of modern phones - they could only display an SMS payload as raw text, they had no functionality to interact with that payload in any way, making zero-click exploits equally impossible.