What's the reason, and why didn't they just use URNs?

The reason is because they're based off the same principle. "ARN" stands for "Amazon Resource Name".

I don't know why they didn't use URNs. Probably because nobody else does.

I think the implication is meant to be "they look like URNs because they are".

Except that they're not; rather, they're ARNs. URNs must begun with 'urn:'. It doesn't make a lot of sense that they're not URNs, but…they're not.

My guess: "nobody uses URNs generally, so might as well save four bytes off every one?"