What you all said for sure, but I think posture has a lot to do with it as well. If you're a pronator who tends to lean forward while standing, then you're really stressing your back body when you're not moving fast enough that your forward lean is an advantage.

That's me, anyway.

Plus, in the specific case of museums and galleries, there's the fact that your brain uses something like 80% of the calories your body burns, and vision uses more than any other sense. Looking at stuff all day is exhausting - even if you're sitting down.