This question is analogous to “why hasn’t anarcho-communism yet worked on a wide scale?” Which is a question with many, many facets to it. You’d have to ask a lot of questions separately.
If I were to try, though, I think the simple answer is “people who work in X area usually do not own the means of production and as such cannot redirect the end product to horizontalist organizations.” Most people can’t just quit their jobs to join a mutual aid group because, without being able to contribute things, the biggest thing a mutual aid group can pass around is time, and most mutual aid groups that exist irl are focused on doing tasks like “picking up prescriptions for others,” and cannot replace participation in the capitalist economy.
Nevermind how most governments don’t want horizontalist non-capitalist organizations to gain enough power to provide a viable alternative to living under capitalism.