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

1. **Poor Planning**: Inadequate planning can result in unrealistic timelines and resource allocation, making it difficult to meet milestones.
2. **Scope Creep**: Changes in project scope, often without proper change management processes, can lead to additional work that pushes back milestone dates.
3. **Resource Issues**: Lack of availability of necessary resources, whether personnel, equipment, or funding, can delay tasks needed to meet milestones.