Because the manufacturing environment is one of constant change. Customers do have emergencies and change due dates or product specifications or need rush orders. Key components don't arrive on time from vendors. Machines unexpectedly break down. Key employees don't show up for work.
Forecasts are wrong.
Equipment doesn't work properly. Quality problems result in unexpected scrap, rework or even failure. The list is endless. A change made in one delivery date can affect all the supporting schedules for subassemblies, materials, and services. Lets face it, if one item doesn't come in on time, all jobs that require that component will have to be rescheduled, all jobs that require a sub assembly that needs that one component will need to be rescheduled and on it goes.