It's a collective action problem like any other. Collusion on a wide scale in competitive markets is pretty damn difficult, since one company can change their manufacturing processes to produce less waste and suddenly undercut everyone else's prices. "Passing the whole price increase onto the consumer" rarely withstands scrutiny, excluding cases with very low demand elasticity or very oligarchic suppliers. Neither of these are the case for the vast majority of packaged goods that are sold.

The incentive to minimize their packaging already exists though, both in shipping costs and just the material costs.