Yes you are missing the fact that work units are not fungible and it is not trivial for a company to calculate the marginal value of employees.

The discourse of "we are paid what we are worth in the labor market" is deeply flawed due to these reasons. So when people buy into that discourse, which advances the capital owning class interests (take what we give you that's your market value) it's both sad and harmful.

Collective bargaining and regulations play a big role in compensation levels and often political capital counts for more then some abstract and impossible to calculate market value of your skill set or work output. So please disabuse yourself of the notion that you have a fair market value as a worker and that you are paid that value. This is pure political fiction.