It’s not a problem if the worker is offering a commoditized service. Problem is a dev isn’t really a commodity. Their value increases the more they work on your application and build familiarity, which isn’t easily replaced. Sometimes a short term dev can construct a system only they truly understand and then you’re entirely at their mercy. Now you’d have to keep an ongoing arrangement with them for future support.
It’s a similar issue with long term devs too. Employers hoping to squeeze their devs for 40 hours a week consistently are going to be very disappointed if they found out how much their devs actually work. What you’re really paying them for is to stick around so when shit hits the fan or you need new features fast you already have the best people for the job ready to go, no need to hire some contractor and go through an onboarding.