I agree. I’m having a hard time thinking of a merger that was actually beneficial to anyone, first- or third-party.
In export-oriented industries, countries often (forcibly) merge all (or many) of their relevant companies into one (or few) companies in order to make them more efficient and internationally competitive. Sometimes it works, sometimes it doesn't.
PayPal is a famous one in tech. There is Exxon Mobil as a big one.
You’re right there are barely any though. Most mergers seem to be bad in hindsight. It’s acquisitions that end up working out better.