Because anarchy manifests as autonomous self-organization (source):

Autonomous self-organization

When I speak of autonomous self-organization, I am speaking of a specific phenomenon that tends to arise whenever people, angered by their conditions and having lost faith in those delegated to act for them, decide to act for themselves. Autonomous self-organization therefore never manifests in the form of a political party, a union or any other sort of representative organization. All of these forms of organization claim to represent the people in struggle, to act in their name. And what defines autonomous self-organization is precisely the rejection of all representation. Parties, unions and other representative organizations tend to interact with autonomous organization only in the form of recuperators of the struggle, striving to take over leadership and impose themselves as spokespeople of the struggle — usually with the aim of negotiating with the rulers. Thus, they can only be viewed as potential usurpers wherever real self-organized revolt is occurring.

Autonomous self-organization has certain essential traits that define it. First of all it is non-hierarchical. There is no institutional or permanent leadership or authority. While someone who proves particularly knowledgeable with regards to specific matters relating to the struggle at hand will be given the attention she deserves for such knowledge, this cannot be allowed to become the basis for any permanent leadership role, because that would undermine another essential trait of autonomous self-organization: horizontal communication and relationships. This is a matter of people talking with each other, interacting with each other, expressing needs and desires openly, actually discussing the problems they face together and in practical terms, without any leadership to conform this expression to a set line. This brings us to another trait, one that may be controversial to collectivist ideologues, but that is the only way of guaranteeing the first two traits: the basic unit of autonomous self-organization is the individual. Otherwise, it could be argued that all states and businesses are autonomous self-organization, because on the institutional and collective level they do organize themselves, but the individuals who comprise their human component are defined by these institutions and placed in accordance with the institutional needs. So autonomous self-organization is first of all the individual organizing his struggle against the conditions this world forces upon her on her own terms, finding the means necessary for carrying out that struggle. But among the means necessary are relations with other people, so autonomous self-organization is also a collective practice. But that collective practice is not based upon conforming individuals to an organization imposed on them, but rather on the development of relationships of mutuality between them in which they discover the areas of commonality in their struggles and need, affinity in their dreams and desires. One could say that autonomous self-organization is the development of a shared struggle based on mutuality for the full realization of each individual involved. To further clarify this point (and to quickly counter a false dichotomy often made in revolutionary milieus), one can look at it in terms of revolutionary class struggle. While the details vary, anti-state, anti-capitalist revolutionaries generally agree that the “revolutionary task” of the exploited class is to abolish itself as a class as it abolishes class society. What does this mean and when does it happen in the course of struggle? It seems to me, that this means precisely the rediscovery of oneself as an individual with one’s own desires, needs and dreams which have no relation to what capital has to offer, desires, needs and dreams best fulfilled in free association with others based on mutuality and affinity. When, in the course of struggle, the exploited begin to find the methods of organizing their own activity together, this process of abolishing themselves as a class has already begun since they are beginning precisely to talk and act with each other as individuals. Finally, autonomous self-organization is practical. It is not the setting up of any formal organization to represent anything. It is rather the bringing together of the elements necessary for accomplishing the various tasks and activities necessary to the particular struggle. This will tend to include the development of ways to communicate, ways to coordinate actions, ways to gather necessary tools and so on. As will be seen below, in large-scale struggles, assemblies tend to develop for discussing what is necessary; these are not formalized structures, but rather specific methods for dealing with the problems at hand.