One other thing to add to your great response. REHEARSAL!

Orchestras rehearse as little as they do because conductors do everything you've mentioned. But additionally they facilitate a singular musical goal, and take the "democracy" out of it. Further to this is the time management. The musicians in the orchestra have enough notes to play and sound to think of, without having to consider how much rehearsal time we need/want to spend on a specific movement, how we would like to spend more time on movement 1 but actually movement 3 is notoriously tricky so we need to have time for it etc. Also, someone has to be monitoring the process to know when enough is enough, it's not always the case that more rehearsal time = better.