Merging the PM with the PO
Question: "Why are some companies merging the product manager and product owner into one role?"
Merging of roles in product management is often done due to budgetary concerns, workplace politics, entrenched traditions, or unsupported interpretations of product management.
The Scrum Backlog Manager role is ill-defined and the Product Manager role is misunderstood, and that just compounds matters. Not helping is that the two roles sound similar, both starting with the word Product.
Exacerbating it all is that Scrum has voids and amorphous language that demand further interpretations. This opens up a scope of possibilities in merging or eliminating roles, without any methodological foundation.
In any case, the Scrum Backlog Manager (aka Scrum product owner) is a role in product development, not product management. It is just a renamed and generalized title for a Software Development Project Manager.
Everything that is based on logic or principles points to not merging the product manager and product owner into one role.