If the party is in a classic dungeon or similar situation, the DM can design the circumstances such that PCs trying to have a Five Minute Workday are going to either be attacked, surrounded, or reinforced against. Tactically, the party will have to face/force a higher number of encounters before camping or else at best end up besieged.
If the party is in the wilderness or the like, there are still random encounters or other cases of encounters coming to the party (if the PCs decide to camp every few miles instead of making a long trek through enemy territory to rest up, what’s to stop large reinforcements from arriving in the area?). Also, even if there aren’t combats going on, the wilderness can have non-combat encounters that drain resources (and conditions that impede proper rest).
The problem with the latter case is that:
A: random encounters are random. You might get attacked, you might not. You might get a short rest, or even a long rest in before you get attacked. But random encounter tables get somewhat
silly. I mean, imagine walking through the woods, and over the course of 8 hours, you get attacked by 6-8
different packs of wolves. Or you get attacked by 3 packs of wolves, two pairs of bears and one mountain lion.
This isn't how walking through a forest actually looks. This is how walking through a
zoo looks.
B: Keying off above, the forest would have to be absolutely INFESTED with whoever you were fighting.
I mean that's fine if you want to present a theme park. But real people can go out into real forests and be lucky to see
one wild predator and a good portion of the time, those predators aren't interested in the people.
If PCs keep trying to rest after just a few encounters, it’s an issue of design more than anything, a design that regularly presents a sense of predictable safety — combat encounters being treated like scheduled sports matches instead of the fog of war. If the circumstances of the adventure and campaign are designed such that simply being awake and keeping moving to clear out space is a better tactical option than holing up, no mechanical changes or benefits are required. Plus it makes for a far more exciting story.
The problem with the logic of "there's always random encounters" or "there's enemies out to get them" is that the logic doesn't magically go away after 8 fights. Once you've set up the idea that the party is going to have to deal with the DM rolling for random encounters
every hour that logic still applies
after 8 fights. If there is a hostile enemy force out to get them, then that hostile force
doesn't stop after 6-8 fights.