The lead is almost never worth what it looks like when it lands. It arrives as a clogged line or a toilet that won't stop running, and the money is in what gets found once a plumber is actually under the house. The water heater that's past its life. The cast iron that's going. The repipe the homeowner has been putting off through two winters. The job that pays for the month started as the job that barely paid for the drive.
That changes what a lead is allowed to cost, and it changes who gets to judge the campaign. A plumbing company tracking cost per call and nothing else is grading its advertising on the small half of every job it produced.
It also changes when the phone rings. The calls arrive at night, on Sunday, and on the coldest morning of the year, which are exactly the hours an answering service is doing the least useful version of the job. A homeowner standing in water doesn't make a second attempt.