"All theaters have a minimum orchestra required. If you don't need that many instruments, you have to hire the minimum anyway and have some guys getting paid to literally do nothing. (You can apply for a Special Situation, but it has to be approved by the union. [title of show] was approved to be a Special Situation.)"
Most shows opening in the last decade have applied and been granted that exception - I think there have been 3 or so requests denied over the last several years, it's pretty much a given at this point.
As far as trying on costumes - aside from the hopefully obvious hygiene issue, many costumes are specially built for the show, to the dimensions of the actor/actress. It would be very easy to accidentally damage a costume by putting on incorrectly, forcing an arm into a sleeve, etc. etc.
Costumes are worn 8 times each week, get cleaned (hopefully

) quite often, and have to endure all the action onstage. Think what shape your favorite clothes would be in if you wore them every day, washed them three or four times a week, and sang, danced, flew and whatever in them. They wouldn't last a month most likely.
All of these things mean that costumes can be fragile at any given moment - It would be terrible for the dressers to come in to prep for the evening's show only to discover that something had been torn the night before. Damage may be found during the day work, but either way it's causing extra work for somebody to have to fix it in a tight time frame.
If/when you're invited backstage - no matter what the person says, do not try on costumes, sit on the furniture, play with the props. That is 100% unprofessional for anyone to offer and if they do, you should politely turn them down.
Contrary to popular opinion, theaters are not playgrounds - they are work spaces that depend on the proper care and maintenance of the equipment integral to the show, including, props, costumes and sets.
And yes, I'm a grumpy old guy.
"It's never too late to have a happy childhood. "
- Tom Robbins