Because no biological molecule can survive past the critical temperature of water, strictly speaking the critical temperature of salt water, which is 5 to 7 kilometers deep depending on whether or not you are in a marine or continental environment. Deepest bacteria = 4,000 feet below the mudline. Deepest archea = 5,000 feet below the mudline. Deepest fossil = 5,280 feet below the mudline. Thunderhawk = 25,885 feet deep. Oops. Abiotic theory 1, fossil fuel cult 0.