I'm by no means anything remotely close to being an expert, but I think that people think they sound better because their mind is so convinced and subconsciously set to believe it will sound better after burning them in. Without them even knowing, their mind is imagining how much better it will sound and when they listen to them after, their mind thinks it does sound better because it forgot what it sounded like before, unless you listened to them for days before a burn in. That's all based off what I've learned in my Psychology class. Either way, even if I only think it sounds better, it will still soud better in my reality, hope you can understand that.
hahaah +1 for paying attention in class.
honestly, whether burn-in is real or not isn't really relevant. if ppl think that their headphones sound better after "burn-in," that's great for em. you can easily "burn-in" headphones through normal daily listening. No need to even leave em playing overnight with pink/white noise or do anything extra to "burn-in" headphones. if you want to leave your headphones playing for extended periods of time to "burn-in," just make sure they are at normal listening volumes (you can blow out your drivers if you leave em blasting for a long time).
just to me, it makes more logical sense to attribute the improvement of sound to your brain adjusting to the headphone's specific sound signature.