1. Two people working a 20h week does not mean the same costs as a single person working a 40h week (think payroll, software licensing, pension admin, healthcare, etc).
So costs rise as a result.
2. A job generally has a base load of admin and toil (communication, on-call, interviewing, PR reviews, etc) and you can't reduce or distribute the base load well enough to yield productive output from the remaining work time with part-time workers and some of this adds more toil when distributed across more people (i.e. on-call requires more handovers during the week and takes more communication).
So productivity is reduced.*
For companies to consider doing this they'd need to perform an trial to satisfy themselves that the above can be overcome. But even doing that may not be possible with existing employees and risks instilling discontent amongst those not in the trial. It's high risk to try.
* Unless the entire company is 100% part-time and somehow base load is fundamentally reworked, in which case #1 still applies.