I think the main mechanisms are probably journals that publish only original ideas, and hiring based on number of publications. These mechanisms have to be counteracted by equally strong mechanisms to spread truth and suppress untruth, but unfortunately in philosophy there is no simple and uncontroversial way to distinguish between these, so the former mechanisms tend to overpower the latter ones.

One idea to fix this is maybe universities could start hiring philosophy professors based not on apparent research progress (i.e., number of publications) but on tests of intelligence and rationality.