Most teams I've worked on had a tech lead, a UX designer and a product manager. Why can't one of the first two people play the PM role? It's not as if being a P

Most teams I've worked on had a tech lead, a UX designer and a product manager. Why can't one of the first two people play the PM role? It's not as if being a PM takes up a person's whole time. In fact, on most of my teams, the PM was also the PM for a few other teams. Having fewer people wastes less time in coordination. I understand there would be some situations where you need a PM separate from other roles, but that shouldn't be the default. Just as most teams don't have an interaction designer, a visual designer, an animation designer, and a sound designer — all those are performed by one UX designer — most teams shouldn't have a separate PM. Is there anything I'm missing?