There are a few problems/issues.
Firstly, the staff at museums and art galleries are usually not into photography themselves. They need to be given a simple set of rules to enforce. The aim of the rules will be to protect the artwork, prevent other visitors from being inconvenienced and to prevent photographers from commercially exploiting the photos they take while at the museum. All perfectly reasonable, but try telling a security guard that he or she has to learn the difference between a Rolleiflex, a nikon F5, Pentax K1000 and a professional full frame DSLR. That's asking too much.
Banning of tripods, monopods and flashes make sense. As discussed already, paintings can be damaged by flashes and the accessories can get in the way of people. If you want to use a tripod, pay for a period of arranged pro photography and you'll probably also be greeted with a gallery empty of tourists.
it is also likely that a lot of these policies haven't kept up with technology. And it is not always recognised that a skilled practitioner can get a great photo with simple equipment, while a bungling beginner with over confidence can shoot hundreds of duds with the greatest cameras ever made. Most flagship smartphones of the last 5 years or so have truly decent cameras, if you know how to use them. There are plenty of compact digital cameras that, with care, can take great photos in low light at museums. But it is not the job of a security guard to know all this. Once again, they need to be given a simple set of rules to remember, and to explain to visitors. Those rules are probably made with the intention of Trevor Tourist creating memories of his visit and Peter Professional not getting a free gig.
That said, I've been to the louvre with a DSLR and film SLR both around my neck and had them admired by the staff. And I've declined to enter establishments where the policy seems to be to ban photography and "encourage" you to buy overly expensive souvenir official photos, postcards, books etc. I've also pointed out, "I might well have bought your book if you'd let me in". I've been to places with the "no changeable lenses" rule which really is out of date considering a good compact digital has greater zoom range than most of us have ever used with any film system.
Backpacks are getting banned in a lot of places, and I don't think it's just because someone can forget and bump into people. It's also concern about bombs. Though one friend certainly does have a backpack wherever he goes with his wife because inside it he keeps her collapsible crutches, medication, medical devices and other items that could literally save her life.
Often when I shoot in museums it's not to own a high quality copy of the artwork or exhibits. It's to look them up later. It's to create nice memories of a day out. It's to share in online reviews "Look at the amazing exhibits here, you should visit too".
In the end it's up to us to check the photography policy before we go, and sometimes ask for clarification by email....if photography in museums is our thing. Mostly, while I think such policies are far from perfect, they are there for valid reasons.....to protect the exhibits, to prevent inconvenience for patrons and to prevent people making money while not providing any revenue for the establishment.