I think there's a disconnect. There's something known as the structure-conduct-performance paradigm. A certain kind of structure will lead to a certain kind of competitive conduct, which will lead to bad outcomes. If it's a concentrated market, it leads to less competition and bad outcomes.
What I think economists have realized over the years is that sometimes really competitive and vigorous competition causes the market structure to be more concentrated. If you have really tough competition, the weaker performers fall away. Then you can look at that and say there's a concentrated market; it can't be that competitive, when in fact it was competition that led to the concentrated market.
The direction is not always the same. A concentrated market is a reason to look further if you have a merger, to be sure. However, it's more complicated than thinking it all moves in one direction.