Why do peanut butter sandwiches become hard and how to prevent that

I've done it before without problems. Even if they're peanut butter & jelly (something that's typically stored in the fridge), it has so much sugar in it that it's inhospitable to microbes.

If you want to play it extra safe, and the sandiwiches won't be easten shortly after you leave, you could place them in the freezer and then let them thaw in your bag.

... all that being said, if you're in the US, the TSA considers peanut butter to be a gel, so won't let you fly with a container of it. I have no idea what their rules are in sandwich form. (I've never tried it, as I typically fly w/ cold cut sandwiches, and just take them straight from the fridge before I go).

update from Erica : "I have flown with peanut butter sandwiches in our carryons. New TSA rules mean pulling it out of bags to send through the scanners, but they had no objections."

The answer is mostly just "it's tradition", as with most questions like this.

I do think the pattern you've described isn't quite the actual one. What really happens is that we tend to eat peanut butter and jelly on sandwiches, and put one or the other on single slices of bread, because with a sandwich you can spread one thing on each half and put them together, but with a single slice of bread it's messy. Sandwiches might be on toasted bread, though - untoasted is common and easier but plenty of people like toasted bread for sandwiches. Similarly, there are certainly lazy people out there who'll just spread something on a slice of untoasted bread.

Beyond that, it's just what we do.