I don't understand why people delay things for latecomers, either. In my dad's family, people are often late, but the hosts go on without them, and the latecomers don't seem to mind. Drives my mom crazy when my dad makes them "late" for something, but as far as I've seen no one else (like the hosts) seems to care. This is for casual family gatherings, not for things with timed starts like a movie.
I could see waiting a long time if you were waiting for just one other person, and it was the first time they'd been significantly late, and you just couldn't think clearly about what to do. Sometimes I don't think well on my feet. But I hope I would realize it was my choice to wait (if I had other options), so that portion at least couldn't be blamed on the latecomer.
I think waiting, at least in a specific situation, is reinforced by restaurants, who for business/efficiency reasons want to get all the orders from one table at the same time. So a server will ask if you want to wait for the missing person/people before ordering your meal, or even assume you want to wait and just ask about appetizers and drinks. I've occasionally told a server that I want to go ahead and order my entree, and they usually seem a bit taken aback.
Here's a question: What if you're part of a group, and the host/organizer/most vocal person wants to wait until the latecomer arrives, and you just want to get on with things? I realize there's a lot of variables in that situation, but would it be rude to just take your ticket and walk into the exhibit, or summon a server and order your dinner, if everyone else in the group seemed determined to wait?