Why is efficiency a defence? Suppose on a given block, there are ten houses, five on each side of the street. Everyone uses propane. To begin with, there are two suppliers, one for each side of the street. But now the firms propose a merger. It’s only reasonable to expect that if it proceeds, the reduction in competition will cause prices to rise. Why have a monopoly if you don’t raise prices? But it’s also reasonable to expect costs two fall.
How do we decide if this transaction is in the public interest? If our metric is the extent of competition, then it’s obvious the merger should be blocked: where there were two competitors there is now only one. The merger has created a monopoly. But we don’t value competition for its own sake, we value it as a means to an end: efficiency. The virtue of competitive markets is that they typically deliver the most value from limited resources.