Because the delivery team is actually expecting a workable product backlog and you're responsible to deliver within your role as a product owner among other duties. If you wanna do something else, then let it be, but the term product owner has a specific meaning within the scrum framework.
Product owner is the person who can make decisions on product features and the development priorities.
Whether they actually arrange the backlog or fire orders or participate in aparté meetings with analysts who then pass the outcomes into the backlog is not essential.
The guide is says pretty much what i say. "How this is done may vary widely across organizations, Scrum Teams, and individuals."
OTOH, The humane, real-world understanding of the word "product" and the word "owner" coming together should prime over whatever definition there is, because none reads definitions except for the purpose of an argument ;-)