I'd imagine another reason is they need the certainty of having their money be there. My wife had an account she didn't use since college, and apparently she fell below a balance limit and they charged a $5 fee, which overdraw her and turned into a $35 overdraft charge. Inconvenient for us, potentially a disaster for someone scraping by.
I had the same thing happen several years ago when I moved. I changed banks and left my "old" account open with ~$18 in it. I completely forgot about it until a few years later when I received a letter telling me they had closed the account and I owed them $9.
