This is the era of huge multinational corporations to which it seems to be whatever that they want from those that ostensibly regulate and handle antitrust issues that these well connected mega corporations almost always end up getting their way. The fact of the matter is that the larger an organization gets, the more concentrated its power is, so that over a period of time, its footprint is large enough that regulators, are typically well outmaneuvered, out-financed, resource challenged, and outplayed, with the message from the mainstream media usually lacking in substance but high in applauding false merger benefits, while much of the credulous public is apathetic or confused.
When massive mega-mergers are first proposed, they are carefully orchestrated events, speaking typically of the need for a particular merger to happen because of synergy, competitive reasons, market forces, innovation, growth, and so forth, of which basically none of these are the real reasons for the merger. The fact of the matter is the real reason for most mergers comes down to money and corporate greed, to which those at the top of the corporate pie benefit, and those that aren't, most definitely come up short and often are dismissed.
For instance, while every big merger is different in its own way, a typical merger, will provide massive fees for those financial firms that broker and finance the deal as well as the legal team that makes sure that all the paperwork and deal making with regulatory authorities is constructed in such a way as to bring a good assurance that the deal will be approved. Further, the top executives of the firms make sure to take care of their own, so that even if some high executives are put out to pasture, as they most certainly will be, that the compensation package that they do received will more than make up for any small damage to their ego.
It can also be said that often real synergies will be created, which in fact, often results in the need for less physical space as well as a reduction in employees because of redundancy, so that a merger of two large organizations, is often the perfect time to jettison employees that simply don't make the grade, without losing much in productivity, which, of course, is quite beneficial to the corporate bottom line.
In some mergers, those that are merging were once competitors, with these former competitors now uniting this creates an entity which is now larger in size and power, which often means that their pricing power and/or their ability to exert pressure on their remaining competitors increases substantially. It can also mean, paradoxically, since there is one less competitor, that pricing can become softer, since the consolidation of the industry, makes it easier to effect a "gentleman's agreement" that prices need to rise, so that the industry can stay comfortably profitable for all.
So too there are mergers that are vertical in nature, such as buying the supplier of your feed for your chickens, so that now you can provide "most favored nation" pricing and throughput for your chickens, while conversely charging a little bit more to rival buyers of such and thereby squeezing your competitors who previously had been receiving their feed from that supplier at free market pricing, but now must struggle with this new paradigm.
The basic fact is that most mega-mergers are done so as to concentrate power and wealth into far fewer hands to which over time, if not almost immediately, that power and synergy will be used to unjustly extract more money and profit from consumers, by small incremental increases. For instance, if you enjoy soda, you might not notice that there really are only two basic choices, in which one will be a Coca-Cola product and the other will be a Pepsi Cola product, with the illusion that all these other different brands, containers, and such, are different companies, whereas they pretty much are a product of either one or the other.