This is quite widespread and unfortunate indeed, and I can think of a few possible reasons:
1. Not getting into the details of the results of due diligence when a rosy picture is presented to them early on.
2. Blind faith in the executive directors such as CEO, and the M&A teams. Not questioning their assumptions.
3. Prestige and empire building aspirations.
4. During early days after the acquisition, they tend to remain patient. This is because, in many cases, a short term downtrend of share value is expected.
5. Many acquisitions have almost become a gamble. With most literature quoting failure rates of M&As anywhere between 50% and as high as 90%, shareholders have come to expect failures, instead of doing their best to change those statistics. This is especially true when the CEO is able to justify the failure by using a change in market scenario or an unforeseen event as an excuse.
6. Beyond a point as the merger fails, sunk cost fallacy kicks in. This is especially true if the root cause of failure is not integration, but the lack of a comprehensive due diligence. Eventually, they become just helpless observers.
