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.
Most if not all of them because they were all friendly deals and the acquired wanted the deal to happen. (In other words, the acquired's owners wanted BigCo's money more than they wanted to remain independent.)
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.
How many add positive value to the acquired?