Incomplete list of problems we are trying to address when we strive for #decentralisation: profiteering, exploitation, surveillance, inequality, hierarchy, intellectual property, inaccessibility, elitism, credentialism, manipulation, conformity, idiocracy, etc...
Or simply put, power and capital.
Annoyingly some restrict their meaning of #centralisation to "I want me some of that too!" (eg. a lot of the blockchain people these days)