It's a law specifically so that these jobs exist. It has nothing to do with liability insurance. Rather, one guy was able to lower his prices by having people pump their own gas and other gas stations didn't like that

Reingold decided to offer the consumer a choice by opening up a 24-pump gas station on Route 17 in Hackensack. He offered gas at 18.9 cents a gallon. The only requirement was that drivers pump it themselves. They didn't mind. They lined up for blocks.

"The other gas station operators didn't like the competition. Someone tried shooting up Reingold's station. But he installed bulletproof glass, so the retailers looked for a softer target - the Statehouse. The Gasoline Retailers Association prevailed upon its pals in the Legislature to push through a bill banning self-serve gas. The pretext was safety, but the Hackensack fire chief had already told all who would listen that Rein- gold's operation was perfectly safe.

https://www.nj.com/opinion/2014/02/the_real_reason_self-service_gas_was_banned_in_nj_corruption_not_safety.html