The current implementation leaves something to be desired, but there are also major fundamental issues within D&D as a whole to contend with.
Probably most notable is just the incredible breadth of what the gamer base considers the normal mode of play. Some people are in dungeons 24/7, others mostly wandering the countryside, others mix&match. Some DMs let you retreat and rest (or the contextual equivalent) whenever you opt to do so, while others press with time constraints or wandering encounters or just the hornets from the hornet nest you just shook up coming out and taking the battle to you. Some groups come with a menagerie of pets, herd of mounts, and retinue of NPCs; while others stick with 4 PCs (and the action economy associated with that).
Second to that (IMO) is the wide distribution in how different PCs perform under different play experiences. Not just 'always on' abilities vs. X/day or Y/sub-part-of-day, but also who does well against traps vs monsters, bosses vs swarms, pure combat vs. everything else, stuff you expected vs. unforeseen circumstances, who is a glass cannons vs <whatever the antonym of that is>, etc. The same goes on the other side of the balance arm, in that some monsters will crash and burn against some parties, while others will be devastating.
There are more issues, but these seem like a huge part of the variability that makes assigning a simple challenge number to an enemy/enemy encounter so fraught.
It's certainly an option, but what does one replace it with? Certainly BitD you had total-HP-available-over-day and spells/magic item uses/day as you do now; but also a much greater risk of just running out of HP right now (since your totals were lower and you were dead at 0 or -10) and save-or-dies (meaning 'still with most resources, but dead so it doesn't matter' was a more common situation). Other RPGs (where combat is a significant component) might play around with one or another aspect -- maybe everything will fully charge within minutes of a battle; or combats are so risky (or healing so burdensome) that pressing on for another fight is nonsense; but in general most have some resource (HP, or the equivalent, if nothing else) that gets expended in fights. Do you have a few examples you were thinking of as models?
I would agree, and 5e's is not the best I've seen, but I've also not seen very many good ones. Wargame unit point costs often are roughly balanced and good at showing 'which will defeat which else, and how easily,' but with a lot of variability. GURPS/Hero System are almost famous for their point values not aligning well with actual power (being more of a fairness/you-can't-get-everything measure than an actual balance tool). TSR-era's HD+asterisks and/or on-what-dungeon-level's-chart-are-they-found measures are nice in that they are so vague no one really relied on them. Again, are there examples we might want to use as comparisons?
The basic concept of 'the adventuring day,' or at the very least the concept of recharging resources and that the GM can either police it somehow or face the consequences of not, have been around pretty much since the beginning. At least unless you did the west marches 'if you leave the dungeon, the DM's other group could well get the loot first' playstyle that man a whole bunch of us never did. PCs have always noticed that they could go out and rest after major or minor resource depletion, and several of the best ways to prevent this (doom clocks, monsters getting wise and reinforcing/leaving with the loot) can strain verisimilitude if done every time (cue debates on how much).
