Urza didn't create Karn
Many people think Urza created Karn as a time probe for his experiments with changing the past. This is partially correct, he got Karn while researching his time machine, but he didn't actually create him. He retrieved him from another plane that he found that was already resilient to the stress of time travel... This whole time. Karn has actually been a Cyberman from the Doctor Who universe that Urza reprogrammed using Xantcha's heartstone.
The metallic human-ish shape matches, the time travel matches, and then when freed from Urza's commands Karn went to create an entirely metallic world filled with metal golem people like him. When that world got overrun by Phyrexian oil that seeped from him, instead of matching the original Phyrexian ideals of perfection being earned and fought for, those New Phyrexians were dead set on "compleating" and transforming flesh people into more of their own, by integrating more metal into them. This is just the typical Cyberman programming worming its way onto Karn's own Glistening Oil.
Oh, and some people in Karn's original plane haven't forgotten about him!
