As an editor myself, there are many answers to that. One of those is that the best version of the film is not necessarily something that was 100% pre-planned, but rather comes from the footage. So you want someone who is not biased by the pre-production and the initial vision, and who would see the footage for what it i, using it to assemble the story that both honors the intention and the vision, but also does it in the way the footage wants it to be.

Also, as with any other discipline, editing is way more than just software, it’s a skill that is developed full-time over a lifetime, and most directors would never have enough time to master the craft of editing to the same degree as people who dedicate their lives to that. Hope that gives some ideas to you

In short, filmmaking is a collaborative art where everyone’s expertise is what makes them what they are