Why Do Most Synergies Fail to Materialize Post-Acquisition?

In many transactions, a significant portion of the deal value is justified by expected synergies. However, in practice, a large number of these synergies are either delayed or never fully realized.

From what I’ve observed, this is often not due to incorrect identification, but rather weak execution. Synergies are sometimes defined at a high level during the deal phase, but lack clear ownership, timelines, and accountability once integration begins.

There is also a tendency to underestimate cultural and operational challenges, which can slow down implementation and reduce the expected impact.

This raises an important question:
Are synergies primarily a valuation tool during the deal phase, or should they be treated as operational commitments with strict tracking post-close?

I would be interested to hear how others approach:

Assigning ownership of synergies
Tracking and validating synergy realization
Managing the gap between expected and realized value