Cast bullets and lube ?

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Why do jacketed rounds need no lube while cast/lead bullets need it? It seems logical to me that a harder jacketed round would need lube and a softer round would not. Can anyone explain this to me?

[MtGun44]

Copper doesn't melt as low as lead, and is much harder and stronger than lead, so it
is less easily wiped onto the barrel. Copper does get wiped onto the barrel, this what
you are cleaning out when you get the blue on the patch with Hoppes or other bore
cleaner. Undersized bullets actually are slightly melted by the hot gasses escaping around
the sides of the boolit. This can't happen with jacketed. Rough bores do build up a
lot of copper fouling, but the jackets tend to smooth out the barrel, too - because
they are harder than lead.

Also, Barnes and other jacketed makers have used various lube coatings to cut friction
and reduce pressures. The relief rings used in the TSX bullets seems to be the final solution,
giving the copper a place to upset to, and lowering the resistance and pressure. Have you
seen the blue coated Barnes or the black Winchester jbullets? These are lube coatings.

Lead alloy actually has a very low friction coefficient against steel, esp when lubed, which
is why all car engine rod and main bearings are coated with a tin-lead babbit alloy on the
surface.

Bill