To keep someone from undercutting others’ prices and causing a competitive pricing war.

Realistically, it wouldn’t be a meaningful drop to the consumer. But it’ll affect some executive’s ability to buy another unnecessary trinket.

If the industry is at capacity (which it plausibly is, especially since HBM memory is made in the same facilities) then no one can physically "undercut" anyone else. Collusion works by artificially restraining supply of some valuable good; if there was genuine collusion at play, we'd probably be seeing companies make less of the expensive HBM (to push its price even higher; note that patent and other IPR restrictions can in fact have this effect, to some extent) and more of the comparatively cheap DRAM!