One reason is that, as a manager, you get an ok to recruit one "head". Not fractions. One. This means, if you recruit someone part-time, you'll have to all effects one person in your team that does 3/5 of the work he/she's supposed to do or you will have to hire another person who wants to work the remaining 2/5. However you see it, this turns quickly into an organizational problem in terms of coordination, effort, communication... I think your best chance to work part time is to work as a consultant and do only the specific hours you need on the projects you're hired for.