That certainly could be one reason, but the constant testing of these lists by hundreds of thousands of people day end and day out would be a bigger one.
The people who post proxy lists, pretty much all leech them from the same sources (each other mostly). They all post them to several forums like this one, and the people who wish to use them test them repeatedly.
Few proxies are operational 24/7, so some proxies that appear to be "dead" aren't in reality. They just aren't non-functional at the time of your test or desire to use them.
