Because in reality, we have to rely on engineers to be honest about their workload and estimates to determine realistic expectations.
The overemployed game is all about exaggerating how much you're working and how difficult your tasks are. Targets, deadlines, and work scoping aren't chosen in a vacuum. We rely on the team to give us feedback about what they can accomplish in the expected full-time work week.
This goes both ways. If I'm handing out 80 hours of work every week, I need my team to tell me it's too much and I'll scale it down. The problem with overemployed people is that they must lie about their workloads and estimates to avoid being assigned more work than they can handle.
We had an overemployed remote person once. Like the parent comment, we noticed when they weren't performing at the same level as their peers (or even a reasonable level of expectation). They were also frequently unavailable for discussions, half-involved in meetings, and generally unreliable.
> For low paid service workers, it's acceptable and sometimes expected to juggle multiple jobs. Why is it unacceptable for office workers?
Low paid workers with multiple jobs aren't doing them simultaneously, but someone with two remote tech jobs is almost certainly trying to do both at the same time. Honestly if someone had infinite energy and could dedicate 8 hours per day to my job and then another 8 hours per day to another job and not overlap the two or run out of steam, I wouldn't care at all. But in practice, it becomes a constant game of lies and sandbagging that is just incompatible with running a fair and balanced engineering team. (Hint: It's often the coworkers who catch on before the manager because they can see very clearly that their peer is sandbagging)