I'm trying to think of what those might be. Assuming you already have employees and a payroll system, I think maybe only worker's comp insurance would be considered a fixed cost. Maybe the cost of a notebook computer or other device, if required. Employees who are less than half time usually don't have to be offered retirement plan options or mandated sick leave/vacation (although here in California, it might apply at all levels, I haven't checked).

Maybe the cost of a notebook computer or other device, if required. And business cards, if anyone still uses those. What else?

edit: possibly some training costs, depending a lot on the type of job and the previous experience of the worker

Office space.

Also it’s just work to hire, manage, administer another enployee. Fiddling around in Justworks one extra time per month. Creating an email address. Etc.

It could be, and that’s a good answer to the question of why we haven’t seen it. It would be nice though, if we made it possible.

I don't think it's so much the costs as it really only makes sense for people who bring specific capabilities that aren't easily replicated and they can apply those without a lot of coordination on a fairly loose and non-intense schedule.

That just doesn't apply to the vast majority of employees.