To answer the O.P. question: Because R.C. members are told to be. R.C. 'priests' and officials teach that their group is the only correct group. The R.C. / Vatican group teaches that there is NO salvation outside of the R.C. system.
Not so. The Church believes salvation is generally not available outside the Church. Thus if a non-Catholic Christian goes to heaven, it is because they've been counted as a member of the Church... their actual membership notwithstanding.
And I don't claim to be an expert on this teaching. Even so, my understanding is that it's not necessarily written in stone. It's more of the Church's educated guess. But ultimately God will make that decision.
If a "Protestant" quotes a Bible verse(s) to a R.C. member --- they either clam-up/shut-up ( because 90% do not know the Bible ) , or if they are on the internet ( such as the R.C. members on this website ) --- they will quote from some "Church Father" (( just as the E.O. folks do to "prove" their point(s). )).
Generalize much?
Besides that, what I surmise from your comment there is that you believe in sola Scriptura. And that's fine but don't you think it's a bit unfair to expect a Catholic or EO to discuss topics exclusively on your terms?
They avoid the B I B L E ,
To tie this in with what you wrote earlier, Protestants tend to avoid the Church Fathers... I think because they tend to write from decidedly Catholic viewpoints. It's tough to read much from any of major names from the 1st and 2nd centuries without tripping over a ton of things the Catholic Church teaches to this day.
Very bluntly I think the onus is and has always been on Protestants to justify their ahistorical doctrines rather than on Catholics. But even there, we can point to writings of people trained and taught by St. John himself as evidence of this or that doctrine to establish the historicity of Catholic beliefs.
and thus they get upset when "Protestants" quote/recite/Post Bible verses.
I can't speak for all Catholics everywhere but I at least get annoyed by it because Protestants insist on bizarre double standards in these sorts of discussions. I'm supposed to adhere to a rigidly sola Scriptura viewpoint... and thus my pointing out that the early Church manifestly didn't believe in that doctrine is, of course, a contravention of sola Scriptura, which somehow "invalidates" my point.
Meanwhile, the Protestant is, of course, free to cite writings from Luther, Calvin, Wesley or whoever else when it bolsters the argument in favor of sola Scriptura.
That's just one example, mind you.
The "Protestant" believer trusts alone in Christ Jesus for eternal salvation , but the R.C. member has been taught that they must be a member of a group founded in Rome.
Whaa?
Incidentally, the primacy of Rome is an ancient belief. Of course, to convince you of that (if such is possible), I'd have to quote from the early Church Fathers... which is apparently a no-no.