How Manuscripts Matter for Knowing What an Author Wrote

In this thread I am addressing the question several readers have asked me about: if I think that the Christ poem of Philippians 2:6-10 is not something Paul himself wrote (as I have argued; see for example https://ehrmanblog.org/how-ancient-is-the-idea-of-christs-incarnation/), but has been quoted by him from some other text, why not just think a scribe inserted it into Philippians? That is, maybe it wasn’t in the letter in the first place; why not thing a scribe stuck it in after the letter...