If the job would take a plumber 30 minutes, it'll take me an hour - so instead of paying a plumber for 30 minutes of work, I'd need to pay him for an hour of supervision; so I'd be paying more for the privilege of doing it myself.
Presumably a single plumber could handle multiple clients, plus you are already paying for his commute, which you probably don’t need to do (in your own house)
If the job would take a plumber 30 minutes, it'll take me an hour - so instead of paying a plumber for 30 minutes of work, I'd need to pay him for an hour of supervision; so I'd be paying more for the privilege of doing it myself.