This is because companies are stupid and lazy when it comes to measuring performance.

There is only one used method of performance measurement: time spent. Every company who CLAIMS to be "data-driven" or "gamifies the system" are lying through their teeth. They're like every other company, they measure performance by hours spent.

I've seen many engineers easily hitting double the number of tickets closed as others. They don't work 20 hours a week. If they did, they would be fired within days.

This is why over-employment is "cheating". Employers don't actually care about your performance, they care about how much you're paying. If you're paying less to them, even if their end of the deal is sweet, they feel cheated. They, like most Americans, value perceived fairness over actual outcomes. They have no issue shooting themselves, or you, in the foot if this looks to be more fair.

I could work two different retail jobs in a day if the schedules work out. I cannot work 40 hours a week as an engineer, much less take on a second job (fortunately I can always find a few mindless meetings to make up my job). So long as I'm expected to work 40 hour weeks my company is justified in asking me not work a second job in my field as I couldn't anyway.

Now I could go out and get a retail job for after my regular job.

yeah, if you're salaried big corps expect to own you. you generally sign away all your creative rights to side projects when you take the offer, and you usually agree not to take other jobs, too.