<disclaimer: have been volunteering for an org fighting food waste by organizing volunteer pickers>
It's simpler than you think.
1. Commercially viable picking actually requires some training (identify pickable fruits, avoid damaging plants, don't half pick)
2. Farmers (rightfully) don't like people who have no knowledge in growing crops hanging around in their fields. They just do dumb things like drink from irrigation pipes (for some crops irrigation is periodically mixed with fertilizer or pesticides).
3. Volunteers are not trustworthy. You have 30 RSVP-ed and 5 show up. Picking season is usually a narrow window of opportunity.
4. Most importantly - the vast majority of people (from the subset of human race who may volunteer for picking in the first place) really don't know how to work consistently for 8-10 hours. They show up 8 AM (3h too late), then they sit down to drink coffee. Your equation is 10 volunteers ~ 1 professional labourer. And then you have to bring professional labour anyhow to clean after them.
All in all, this doesn't work.