Several reasons. 1. Absolutely incredibly complex architecture. The recommendation engines are not just single libraries that can be easily open sourced or tra

Several reasons. 1. Absolutely incredibly complex architecture. The recommendation engines are not just single libraries that can be easily open sourced or transformed to a plugin system. 2. Recommendation engines exist to push up relevant metrics. These are either clear wins for users (like reported satisfaction), mixed wins for users and businesses (content engagement), or clear wins for businesses (ad engagement). Most businesses aren't thrilled about subbing in systems that degrade metrics.