why hard cast bullets

[Jack]

I'll offer a slightly different viewpoint.
Hard cast bullets work very well, and won't cause much leading- with an important proviso. For a hard cast bullet to work well, it MUST fit the bore precisely. Ideally, it should be a tiny hair over, or at, bore diameter. If you try firing hard cast bullets that are a bit undersized for the bore, you'll discover they can lead quite a bit.
Soft swaged bullets can work very well, for what they are best at- which is low speed, wadcutter type loads. Soft bullets will be more likely to 'slug up'- that is, be expanded to fit the bore when hit by the forces of the powder gases.
Hard bullets don't 'slug up' as easily, so they must be a more precise fit in the bore when loaded to work well.
Keep in mind that leading is caused when the the hot powder gases get up along the sides of the bullet, and begin melting it. When the bullet really seals the bore, that doesn't happen.