That is what I also thought. Official low CR Monsters are Minion Type... Maybe a little more differentiated approach would have helped here insead of One guideline. The DMG guidelines seem to present a more 4e design approach. Low damage, high hp. fights that last for a while.
I would have liked offensive CR and defensive CR spellt out in the Monster Stat block. Would really help to create fun encounters.
Also 2 kinds of Tables in the DMG, one for more offensive monsters, one for more defensive monsters would also work. Not that I believe it is terrible or so, but the CR 1/8 HP range 7-30 is way too high. 7 HP, down in one blow. 30hp, Soaking up a complete parties attacks for one round. Makes a very very different kind of creature. And in my opinion not granular enough.
Most CR 1/2 creatures have less than 25 hp. So I´d like to have CR 1/8 at 7-14, CR 1/4 15-30 CR 1/2 31-45 or something like that. Having that big jump at CR 1/8 seems not like a good approach...
Even with the low damage, high hp creature, fights don't drag. 3 or fewer rounds on average is a pretty quick fight.
While its true that the stat block when viewed on its own is very static, the actual guidelines that accompany it provide a wide array of directions a DM can go. They don't focus on a 4e design approach in my opinion because they allow you to create the "average" creature or something that does more damage. You need that big HP jump between CR levels because that and DPR are the currency you spend to add traits and other special stuff. If the DM does not modify the HP or DPR as other powerful abilities are added, the creature ceases to be a CR1 creature.
The HP of creatures in the MM are lower than the stat block in the DMG because the creatures are modified in other areas as well. If the DMG tables had CR 1/8 at 7-14, CR 1/4 15-30, CR 1/2 31-45 then the base AC, DPR, and AB would have to be higher, and not all creatures in the MM are built that way. The other thing is that the stat table is listing
effective HP, AC, DPR, and AB. Traits and other special abilities modify these in a way that will not be displayed in a creatures stat block in the MM.
Perhaps even more compelling of an argument is that having the high HP in the table allows for the AC, DPR, and AB to scale from low numbers at CR < 1 to high numbers at CR 30. A CR 1/8 creature with 7-14 hp falls within the existing guidelines, although it is a bit on the low side. A CR 1/4 creature with 15-30 hp would need an AC of 15, a DPR of 7, or an AB of 5 to balance out the lower HP. If the lower HP is codified, then at least 1 of those changes would need to be codified as well. The DPR would be the best candidate from a standpoint of keeping the AC and AB scaling from low to high, but it makes the creatures more swingy by default. The higher DPR and lower hp would continue all the way up to CR 30 to provide a constant progression. This could be done, but as I mentioned before, it might make the creatures even more swingy if that is the default that everything is built off of.
It seems that HP more than anything else is the currency used to purchase, higher AC, AB, DPR, traits or other special abilities. The end result is that the HP in published monsters is lower than the effective HP listed in the CR table in the DMG.