Why do so many non-Catholic/Orthodox Christians view them as “not Christian”?

I don’t want to generalize here. I know not every person of xyz denomination is going to hold the same beliefs on this. No denomination is a monolith and, disagreements and all, we are all siblings in Christ.

Also, when I say Orthodox, I mean both Eastern Orthodox and Oriental Orthodox.

As I’ve read more of the Bible and see how much Jesus calls for a unified Church, I’m baffled by the way I’ve seen some more modern denominations attack them to the point of calling them non-Christian or even pagan. This isn’t limited to the online sphere; I’ve encountered plenty of anti-orthodox (little o) sentiment from Baptists, Pentecostals, and non-denoms in my life, especially having grown up non-denom until I hit high school.

Admittedly, I’m a Catholic myself. I’m not necessarily looking to defend my own side of things nor am I looking to change my faith. However, I do want to understand where the divisions are.

Maybe I’m just misinformed because I’m still new to all of this, but I don’t really see this issue in my interactions with “high church” Protestant denominations. I’d argue they’re even more open to ecumenical dialogue compared to what I’ve seen from the usual Bible Belt/Deep South suspects.

Not looking to cause a flame war; just looking to understand the position better for my own day-to-day life.