Bad M&As happen all the time. Good ones are the exceptions not the norms.
"Companies" are groups of people, and their leadership.
What the leadership wants very often has nothing to do with the company's long-term future.
This is all observable behavior happening all around us.
Is that really true? Here are lists of acquisitions by Amazon and Google:
+ https://en.wikipedia.org/wiki/List_of_mergers_and_acquisitio...
+ https://en.wikipedia.org/wiki/List_of_mergers_and_acquisitio...
The vast majority of acquisitions are uneventful and added positive value to the acquirer. Maybe you're thinking of high-profile multi-billion dollar failures like Daimler+Chrysler, HP+Compaq, Microsoft+Nokia. It's possible that the amount of negative press those failures receive distorts the perception of the (quiet) successful acquisitions.
How many add positive value to the acquired?
Most of the targeted businesses were privately owned which means that the big acquirer cannot execute a hostile takeover and buy the company against the Board of Director's wishes. Amazon's acquisitions of Zappos shoes, dpreview camera website, etc were all friendly deals. Same situation with Facebook acquisitions of Whatsapp and Instagram.