Is there a reason why, at least in the US, that:
1) on dry bread, we typically eat peanut butter AND jelly, not one or the other;
2) but on toast, we typically eat either peanut butter OR jelly, not both?
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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.
Within my household, growing up, I often had just peanut butter in a sandwich. Just jelly generally didn't happen for a simple reason, that without the peanut-butter first placed on the bread, the jelly would soak through. The same logic was present for peanut butter and honey sandwiches, or jelly and cream-cheese, and this was why I generally never had a tuna sandwich packed in my lunch unless there was lettuce protecting the bread.