"It is the hardness of the metal."
That's the core of it. Sorta.
Carbide is almost as hard as diamond and has a very low friction factor. Plus, for straight wall cases, it only makes contact in a quite narrow ring so the friction is restricted to a small point.
Dillon sells very costly solid carbide sizer dies for .223 and .308 for use by commercial reloaders (well, they will sell the things to anyone wanting them). Those full length rifle dies require lubing same as another others.
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