I’ve come to terms with this (in bold) recently myself, actually; I was pretty dang high on my hobby horse.
You are correct, it is wrong to be prideful, and it is incredibly hard to be aware of that. I can sadly count myself among those numbers who acted as such, but I’m trying my hardest to step back and see myself in their number. And thank you, you’re correct, not all Catholics are like that. But as a fallen one myself, I can speak to the Catholics who are "holier than thou"ish. I’m not really sure why we do that aside from sheer human pride; I know it’s not church teaching to act like that! So frankly, I would say it’s just weakness. It’s a flaw that many of us, of all religions, have to face.
As C.S. Lewis said in Screwtape letters, God didn’t make us at varying levels so that we could disdain one group or another; He gave us varying “levels” of holiness so that the ones who were farther in their journey could humbly help those whom they could, and put them IN FRONT of themselves, and so that the ones who were behind in their journey could humbly accept help. It’s all a cycle of humility, and it’s difficult to operate without humility, which is all too common nowadays - myself included.
And for those who are going to pick up the “but some sins are still worse!”, sure, but who are we to really say? Yes, somebody who commits murder and is happy about it might have a special place in hell, but they might repent before their death, while another person might just tell little white lies and never confess those and end up in hell anyway. Again, humility; we aren’t called to
judge the sins and call ourselves better than that; we’re just called to see all sins as evil in the end, and do God’s will to help those who need it, and ask for help ourselves.
We’re all in this together, guys.

Thank you for bringing this up, Philomena!