I think there are two major reasons.
1. Because so many plumbers do rip people off with shoddy workmanship.
2. The role Plumbers have played in fostering the dishonest and confusing use of the term "labor" in relation to billing.
TRUE Story. Look it up Clarion ledger Jackson Ms
old plumber, had a company called PTL [Praise the Lord]
He went to a fellas house to fix a leak under the house. he fixed it
handed the old Dude a bill.
He said I aint paying the damn Bill, If you had fixed it right the first time
It would not be leaking now!!!
Plumber, looked puzzled, asked him you sure it was me?
old Dude, Hell Yes, 15 years ago you fixed it.
if you had done it right, It would not be leaking!!
Plumber said, sir, You have to pay this bill, Or I will put a contractors lean on your house.
Old Dude, Shot him, DEAD.