Some of the amounts are often quite small. I'm sure I probably have an old account from my student days with a fiver inside in it that I may never have closed. Likewise, there are probably many non-nationals gone home, Irish people who emigrated etc and all of those €5 add up.
You're correct as well on people who are deceased. Speaking as somone who was once the executor of a will for a bachelor farmer, we found a few accounts that hadn't been touched for years.