Why do the DMF bolts need to be replaced?

Not all Flywheels come with the bolts..
....while others have them included in the box.
Due to continuous, strong alternating loads, specially designed bolts are used to secure flywheels. These are usually stretch bolts or bolts with microencapsulation. Stretch bolts have an antifatigue shaft that only covers around 90% of the thread core diameter. When tightened to the torque specified by the vehicle manufacturer (in some cases plus a fixed angular value) the stretch bolt becomes an elastic bolt.
The resulting torque is greater than the external force acting on the flywheel during operation. The elasticity of the stretch bolt means it can be stretched right up to the elastic limit. As they do not possess these properties, normal set screws would break after a certain amount of time due to material fatigue, even if they were strong enough in their design. Bolts with microencapsulation (these can also be stretch bolts) seal off the clutch chamber from the crankshaft chamber filled with engine oil. This is necessary because the threaded holes in the crankshaft flange are open to the crankcase.
These coatings also have adhesive and gripping properties, eliminating the need for any further bolt retaining devices. Bolts that have already been used must not be used again. Experience has shown that these snap when they are tightened. In addition, their sealing and gripping properties are no longer effective.