It is not (only) a social contract thing. It is a game design thing.
D&D 5e in both versions give DMs RAW only two options to challenge a party combat wise:
4 deadly to 8 medium encounters in a day (or 15+ rounds of battle) or having one or two double deadly encounter.
Anything else is challengeless. One or two combat encounters in a day that are not double deadly? It will be trivial. The party will blow trough it.
But at the same time, you don't want your random wilderness encounter on the way to the dungeon to be double deadly, but not making it double deadly makes it meaningless, because it becomes trivial.
The ressource reset after an 8 hour long rest is, what hinders other kinds of challenges. It is a big pacing issue and raw 5e only supports one pacing: 6 to 8 encounters inbetween long rests. Nothing else is really supported. That is the problem with the current rules.