On an airgapped system that is is turned on once and needs to keep running for many, many years? Industrial equipment is a world of its own and internet best practices just don't transfer directly.

Some PLCs run extremely expensive machines. Some machines can't afford to have their control systems stutter or fail because that can lead to physical damage and production outages of enormous proportions. A PLC that stops communicating because a certificate just expired is absolutely not acceptable in some plants.

Running certbot on your web server is easy.

Running certbot on a random PLC isn't happening.