Price fixing omits the consumer from the table. It is a way to strip the consumer of the power of choice with their dollar, to avoid competition in the market AND price gouge at the same time, and done so in secret to boot.
Union negotiating includes company representation/owners. They are part of the conversation and agreement. They are a way to more evenly share power with their employers and have a say that they are otherwise not given.
I know people like to think that a consumer spending their dollars is the same as them agreeing to the price. But when the things that they are spending their dollars on are food, Healthcare, housing, education, fuel, utilities and other basic necessities that consumers do not easily get to abstain from… that is not consent, that it coercion.