Because if a thing is valued by thing-consumers at x and you set the price to <x, then you are incentivizing people to use more of the thing than they need, even to waste the thing. This thus requires more infra than is actually needed or wanted.

This doesn't go away under socialism/communism/collectivism. If you set the price too low, you either have to build far more production capacity at public expense than needed, or you cope with regular blackouts.