Why do noble metals like silver not form natural cubic crystals?
Native silver crystals can be found in cubic, octahedral, or dodecahedral forms. As you mention, silver often is found with gold, copper and other metals, and with gold, tends to hexagonal stacking, rather than cubic crystals.

Silver may also form pseudomorphs, in which the crystal shape is from its progenitor or its surroundings. For example, dyscrasite, $\ce{Ag3Sb}$, or acanthite, $\ce{Ag2S}$ may be replaced by silver in the form of the antimony or sulfur minerals.