I mean, isn't that (one of) the point(s) of the corporation as a legal entity: to allow people with capital assets and a joint enterprise in mind to coordinate their resources and act as a single entity for the purposes of pursuing that enterprise [edit: which of course includes purchasing labor in all but the smallest undertakings]. One of the original motivating theories behind the drive for labor unions organized within companies (rather than earlier forms of labor organization like guilds and professional societies which predate modern economies) was that the unification of capital interests within a company necessitated that labor similarly unify, since the alternative would be that the individual seller of labour would lack any kind of bargaining power when negotiating with their employer.

Unless we abolish corporations as a concept, how exactly do you effectively propose that we "outlaw all cartelization" in a way that isn't just outlawing unions while keeping capital interests unified?

tl;dr: unions complement the inherrent concentration of capital interests in modern ecconomies and are no more inherently like cartels than the corporate form itself.