I know each release comes out with new features and improvements that probably cause further battery drain. But has anyone looked into the specifics of why this happens year over year?

Couldn't they spend time optimizing battery life? Is there no incentive to?

Especially with screens becoming relatively larger and batteries remaining the same, I think battery life is a valid selling point for many consumers. I'm sure Apple does a ton of market research in this area, so maybe I'm wrong.