Why do people say "Catholics and Christians" in (USA) when Catholics are also Christians, as if they refer to it as a different religion.

Is this some sort of remnant of evangelical puritan protestant ideology?

I don't understaun this.

If you ask me, it'd make as much sense as Orthodox and Christians.... or Shia and Muslim...

I know not all Christians are Catholics but for feck's sake...

They're all Christians to me....

Edit:

It's a U.S thing but this is the sort of things I hear...

https://www.gotquestions.org/Catholic-Christian.html

I am a Catholic. Why should I consider becoming a Christian?

I now know more distinctions (apparently Catholicism requires duty and salvation is process, unlike Protestantism?) but I still think they're of a similar branch (Christianity) so I just wonder the social factor