I think this issue is somewhat more complicated. Is the client willing to pay for skilled engineers at a true labour cost (as most labour charge out rates as less than the true cost) Are the clients willing to have their equipment off long enough to allow the engineers to complete a thorough test. Generally no, so the contracting company sends less skilled engineers who are cheaper to employ. If a system has a slow leak, then this is good for the contractor, as is gives extra turnover and hopefully profit.
Then you have the upkeep of leak detection equipment, we all know how troublesome electronic leak detectors can be.
And then you have the times you have done a 18hr day, you can not find a pissa, so recharge, with all the best intention to go back, and never do.