It's only been like 10 years and he's forgetting things, chopping off his own body parts and going quite insane. By the time he even caught up to the Ripley timeline, he'd probably have the intellect of a gumball machine.
Playing Devils Advocate: Unless the process for maintenance was automated, or there were qualified personnel onboard covenant that he could force or manipulate into doing it. Somebody had to take care of Walter, afterall. Notice how I cleverly dodged saying "service" there. Unless he could find a way of being able to put himself in cryo. But then there's that ugly road of turning himself human, no doubt by harvesting some of Shaw's organs and trying to give himself a real heart or something.
But all that aside, he's likely losing his mind at a steady rate. Hell, even Ash went nuts in record time, and he should've been manufactured long after David. Ripley's story has been told, there was no David in it. By the time he reaches Ripley 8, I'd half expect him to be a smoking pile of wires.
