Full bushing chains last significantly longer than bushingless chains,,

Now all derailleur chains are bushingless chains ..

The function of a bushing is taken up by punching the metal in the inner side plates into a flange
that only supports the ends of the pins, and the edges of the rollers ..
and the wear is concentrated on a smaller surface..

& part of making thinner chains is thinner side plates, so thinner partial bushings.

cant harden them separately, separate bushings can be ..

But that is the chains for just iGH and Track Use these days.