Time - Communicating with potential referring domains, analysing their link profiles, writing the content itself and ensuring that the article gets published in time usually requires a lot of time and effort, unlike on-page.
Budget - To save some time, it is also possible to cooperate with outreach agencies which already have a robust database of partners. However, such services are very quite costly and it is also important to closely monitor the quality of the delivered outputs on a continuous basis.
Performance evaluation - Many times, a new backlink was not the only SEO adjustment in the given period and attributing a performance increase to a single outreach campaign or on-page optimization might be very tricky.
In the meantime, there might have been many changes that might have affected the performance, such as organic backlinks without contract, negative SEO campaigns from competitors, on-page optimizations and technical changes from other departments and most importantly the neverending Google algorithm updates.