The FTC and DOJ don't block mergers because it may result in a monopoly. They don't specifically judge things based on monopoly or market share, but rather based on whether it will likely or has resulted in consumer / market harm (and I would agree that a quasi-monopoly merger is far more often than not going to fall into that category in the eyes of the FTC or DOJ, but that's not the criteria they start from).

Intel for example has / had a practical monopoly in processors for many years in PCs. The government was ok with it, based on various terms they agreed upon with Intel. The same is true of Microsoft, and countless other businesses (big and small) across the US.

There of course used to be far stricter rules on local monopolies in media (radio, tv, newspapers, cable stations etc), but those have been heavily relaxed in the last 20 years.