Some workers are pretty interchangeable. If Sarah can assemble parts at the same rate as Sven, then the employer shouldn't care who does it, right? However, for specialized positions or ones that are knowledge-based, this doesn't work. Sarah is a lawyer who's been working on a case for the past 3 months, so it will take her much less time to complete the next step than Sven, who has been working on something else. So in that case, the employer would be pretty unhappy about Sarah wanting to work 30 hours a week, even if Sven is willing to pick up the extra 10.

Software developers with the same skills working on the same product are generally closer to the former than the latter, but it depends.

The other thing is that there are many fixed costs associated with hiring an employee. They need a desk and office space and computer, they need health insurance/unemployment, you need HR to manage all of this. So it ends up being much more expensive (and complicated) to have two part time employees than one fulltime one. If most benefits were decoupled from employment, a lot (but not all) of these costs go away.