Now I get that any DM can house-rule (and I'm curious who does) special situations.
You don't even have to house rule it - this concept exists in the DMG, iirc.
Those of you who are used to older editions, what justification to you use for nightly healing? Or if not, do you have your own house-rule?
Okay, so, there's in-game, and out-of-game justifications. The latter is, as far as I am concerned, far more important.
In older editions, playing the healer was, IMHO, not a whole lot of fun. You had to reserve large amounts of your power for healing, just in case. You spent a lot of your time swinging a mace to little effect, and then healing up the folks who had been of great effect.
So, one out-of-game justification - that it allows those with healers to not sit on their hands all the time. Another is that I personally find actually interacting with scenes to be where the fun in a game lies. Making it hard to heal up reduces the average number of scenes we'd get through per session - so, there's another out-of-game justification in that it allows us to actually play more game, with less fiddly hit-point bean-counting.
The last important justification is that... we are playing pretty pulpy fictional heroes. Go watch Die Hard again. Beloved action genre staple, right? But, there's no way that hero should actually be up and about at the end of it. That's okay. It isn't about being realistic, it is about HEROISM and ACTION. Suck it up and deal.
IN-game, we can lay about ourselves with rationalizations about hit points not being meat and all that. But, to be honest, if you can't accept the primacy of the out-of-game reasons for the choice, those won't help you. And if you can accept the metagame rationales, you probably don't need in-game rationalizations.