The conductor as part of the rehearsal is always overlooked here. Music, especially ensemble music with 80+ muscians, can easily be interpreted in many ways. Even when the sheet music has guidance, there are always degrees to decide on. How fast to go here, what volume to play here, on and on, and generally every instrument has different requirements to make a piece fit together. Someone guiding that process, having an understanding or at least a point of view, is a big part of the job. The stick wagging is just the end point after all the other work is done, but it in itself is also useful for execution and consistency.

In concert, jazz, and marching band, I was always grateful for the conductor as a visual cue for timing. All of that "dancing" often tells you where you are in the music or how you should be playing. I briefly performed in a rock band and tried using a metronome in my monitor to keep on beat. It was so much more difficult than I ever realized and I had to have a recording of our music played into my in-ear monitor so I wouldn't get lost. What I would have given for a dancing possessed muppet rock conductor.

Conductors (drum majors) are absolutely vital in marching bands. Marching bands (both traditional military style and modern style) use formations so large that the speed of sound is a significant factor - trying to keep tempo based on listening to e.g. drums simply won't work, because different positions will hear the beats at different times (and that's not even considering issues like echoes, or the auditory dead zones that closed structures have). Having visual cues for keeping time is a necessity.

You 100% need a conductor.

Light travels a lot faster than sound.

So even if everyone focused on a single musician they'd all slip out of time.

Even with a consistent, steady tempo, someone needs to set it and keep everyone the same. For rock and jazz bands, it's usually the drummer, but an orchestra isn't going to have a steady drum rhythm that everyone can hear.

It's not about the complexity of the music, but the logistics of getting lots of people playing in sync. A large orchestra is large enough that there will be fractions of a second difference between the more distant players. In some cases, the players literally cannot synchronize themselves via audio cues alone.

Also, good luck getting dozens of people to come in from silence at the exact same moment without a conductor. They're pretty critical when they're needed, which is very often for orchestras.