That's not my work

I'm so tired of the phrase, "it's not my work".
A/c installers wont do anything that might resemble electric work.
Plumbers won't do anything that might resemble carpenter work.
Carpenters won't do anything that might resemble glass work.
Painters won't do anything that might resemble carpenter work.
Yet I have to dabble in it all as I go behind each different worker to make sure their job is done correctly and thoroughly.
What am I paying them for?