Aren't ETCs also generated from unused travel funds? Like I book a ticket for $700, I change the ticket and pay a $200 change fee but the new ticket is only $675, I end up with a $25 ETC and it gets lost in the swamp of life.
I bet most people who get a $1,000 ETC find a way to use at least part of it. But $25-$100 ETCs (or unused portions of ETCs) can be hard to use if you are price sensitive and other carriers happen to be cheaper, and for many people the one year expiration date means they only have one or two trips to use it on. Add in foreign travelers who don't see United metal often and people who can't keep track of things (most people)...