Hypothetically, imagine a rule in the book where the first four encounters in a day were only worth a quarter of their listed XP, and all encounters after the first four were worth double the listed XP. On average, if you had seven encounters in a day, then you'd get XP equivalent to the listed XP for seven encounters; if you called out before that point, then you'd get much less, and if you went longer then you'd get much more.
If the game world actually worked that way, then people living in that world would notice. They would observe that the cautious adventurer, who gives up as soon as things start getting tough, would not advance as quickly as one who kept going as far as they could on a daily basis. Given that the latter would advance at about seven times the rate of the former, anyone could ask them how they did it, and observe that data.
In our real world, skill and experience aren't quite as important as they are in the game world. They're still important, but it's balanced by the slow accumulation of injuries (which never heal quite perfectly) that eventually force people to retire. There are games which attempt to model that reality - GURPS and Traveller both come to mind.
D&D doesn't work that way. The rules aren't trying to model a reality where people break down and become more decrepit over time. The rules are trying to model a reality where cool heroes get into 6-8 battles-to-the-death every day without it being a big deal. That's the intended design goal. But the rules fail to meet that goal, because they don't give the characters a reason to have so many battles in a day, while they do give the characters a reason to call it a day after just one or two battles.
Given that XP and levels already don't work realistically (giving too significant of a benefit, relative to physical deterioration), adding in the hypothetical rule I mentioned above would not make the world any less realistic, but it would encourage the behavior that we're looking for. (Rather, it does make the world less realistic, but it does so in specifically the way we want it to be unrealistic.) If we just say that this is a world where achieving superhuman competence at arms requires pushing yourself to your limit, then everything is fine and internally consistent, without any need to meta-game.