Because there isn't a conversion. Catholicism does not guarantee regeneration, consequently, the overwhelming majority of RCs are unregenerate. And there is nothing worse than being unregenerate but thinking you can mix works and faith and offer up something to God that will please Him.
Gal 9:30 - 32 explains the RC dilemma very well ...
30 What shall we say then? That Gentiles, who did not pursue righteousness, attained righteousness, even the righteousness which is by faith;
31 but Israel, pursuing a law of righteousness, did not arrive at that law.
32 Why? Because they did not pursue it by faith, but as though it were by works. They stumbled over the stumbling stone,
This is something you could not possibly know.
> There's still a Commandment forbidding this kind of talk. It's called "bearing false witness against your neighbor."