CapnZapp
Legend
That's because your theory leads to at least one boring utterly trivial encounter each day, guaranteed.Really, it's more a 3e thing to have single, lone monsters be a challenge for the party. Earlier editions, this certainly wasn't true. Very, very few monsters straight from the AD&D MM would be much of a challenge for a group, unless you went really far over the party's heads (bombing Ancient Huge Dragons on 1st level parties for example). Heck, the first Dragonlance Module had an Ancient Black dragon as a challenge for a 5th level party. And, even then, the party was expected to win that fight with minimal losses, after having traveled all the way through an enemy city.
3e changed that by having one creature be balanced against a party of 4, where CR=Level. It led to a system without a lot of granularity. As was mentioned, as soon as you went up into six plus baddies, the baddies had to be so weak individually that they stopped being a challenge. Then we have 4e which rejects 3e's baseline in favour of where a given baddie was equal to a single PC. The advantage of this is a lot more granularity. You could jump up a single monster into a Solo and have it be a fair challenge for the group, or use multiple elites, or normals or minions (which split out about 4 or 5:1)
5e has adopted a balance similar to AD&D and 4e where a single monster isn't really meant to challenge the entire group. Where 5e is a bit more fun, IMO, is that the combats are much, much faster than 4e. Four rounds or so usually catches most combats. A 5e monster by itself just can't do enough damage in 4 rounds to really be much of a threat. But, where the balancing factor comes in is in the number of encounters. You're supposed to have two or three encounters between short rests.
That first troll you meet when you're fully rested is going to be a pushover. It really is. But, that second one you meet twenty minutes later is a lot more challenging because now the fighter's can't Action Surge and it's possible that the casters have blown out some of their bigger spells. That third troll you meet shortly later though. Now that one's going to put the beating on the party. All the short rest recharge characters are out of gas and the casters may very well have blown all their big guns already. Now you have to deal with that challenge using nothing but normal attacks and at wills and then you get the real challenge.
I think that people are not pacing encounters often enough and that's why we're seeing this idea that lone monsters are so weak. Against a fully rested party that can blow all its resources? Sure, you can punch WAY over your weight class. Against a party that's gone through a couple of encounters first? Now the fight gets a lot more interesting.
Of course the system must be able to support the fundamental trope of "BBEG single-handedly trouncing the merry band of heroes", anything else is most definitely a bug, not a feature!