It's less about rider safety and more about trackside worker safety. Even platform barriers are more about protecting the train drivers from the trauma of someone stepping out in front in a suicide attempt than protecting riders from doing so.

It's one more thing they'd have to maintain and reinstall when doing track work. I've heard stories from the inside about parts being deliberately left off trains after being repaired that don't risk safety or operation, but will protect the mechanicals from premature failure and weather corrosion. I'm told they leave them off because "it's easier."