"Why do people insist on calling it the American Church, the French Church, etc."
Its a little more than semantics. Amchurch has its USCCB, which voted overwhelmingly to leave it up to the individual bishops to violate canon law and give Holy Communion to public facilitators of abortion, to turn Ascension Thursday into Ascension Sunday, etc.
Canada has its own CCCB, which makes its own rules, etc.
Prior to VC II, Catholic meant universal. New Church, however, defines Catholic as diversified.
Heck, nowadays you can even be catholic while remaining outside the Catholic Church.
The one connotes a unified,universal Church wherever it is located,it conforms with our creed,one can recognize that the Church is One,Holy,Catholic and Apostolic.
The other defines the church by its location,not good nor does it seem true.