Because places of worship in Christianity are usually called churches and Christianity is the most common religion in the United States.
So if I was going to pick a single word used to represent places of worship when describing such in the United States, “churches” would be a good choice.
>If it's no big deal, just post the same comment with the word mosque or synagogue.
Why only "mosque or synagogue"? Are you singling out Abrahamic religions? What about shrines and temples? Words have meanings. If it's no big deal, just post the same comment with the word "places of worship" to be inclusive. :^)
So if I was going to pick a single word used to represent places of worship when describing such in the United States, “churches” would be a good choice.