Milestones in project management are often not met due to several reasons, including:

1. **Poor Planning**: Inadequate initial project planning, including time estimates, resource allocation, and scope definition, can lead to unrealistic milestones.
2. **Scope Creep**: Changes in project scope after its initiation can increase workload and resources needed, causing delays in meeting milestones.
3. **Resource Issues**: Lack of necessary resources, whether manpower, materials, or technology, can impede progress.