No idea why they lived so long but I would move them around wedge them in the rocks. To tell you the truth they wouldn't die, I even tried to get rid of them. At the time no one wanted them.
But my tank is full of sponges, enough so it has become a problem.
I do not run any chemicals, carbon, ozone, phosphate removers, just the absolute basics. Skimmer,CA reactor, LR, Fuge, sand and flow.
Maybe I should give a few new ones a try.