I see the same problem on construction sites. The pressure for production starts from the planning and control system, and is transmitted to the crew by supervisors chasing reportable progress. Doing the easy work first makes the progress reports look good but destroys the orderly and predictable flow of work. Profit drains away as people work in roundabout ways that are hard to imagine if you don't stand there and watch. This problem is not caused by stupid people.
Cost controls hide the waste they create by making workflow between crews unpredictable.
It's an uncertain and crazy world. Evidence shows that current planning systems themselves also cause unreliable workflow because assignments to crews are more dream than plan. If you improve the reliability of planning, projects get done sooner, for less cost. Measuring planning-system performance is the only way you can tell if your planning is working. Improve its performance and you can tame the dreaded trade-off between schedules and costs.