I'm wondering why 5e made NPCs d8 for just being medium size, while classes get d6, d10 or whatever. I kind of give more HP to NPC fighter-types than mage-types, but it still does not make sense.
The idea of all medium-sized creatures use a d8 for base HP was part of my source for the OP.
Because NPC aren't really designed using classes. You
can, of course, the but idea was you didn't have to and it was supposed to make creating such "monsters" easier. The only adjustment otherwise is how many you get.
A Scout (CR 1/2) has 3d8
An Evil Mage (CR 1) has 5d8
A Spy (CR 1) has 6d8
A Bandit Captain (CR 2) has 10d8
A Druid (CR 2) has 5d8
A Knight (CR 3) has 8d8
An Archer (CR 3) has 10d8
A Martial Arts Adept (CR 3) has 11d8
A Gladiator (CR 5) has 15d8
A Conjurer (CR 6) has 9d8
They already give more d8s to many fighter-types, such as the Bandit Captain (10) vs. the Druid (5), or the Gladiator (15) compared to the Conjurer (9), for equal CRs.
EDIT: Adding some of the highest CR humanoids that seem "class-based" in concept anyway:
Archmage (CR 12) has 18d8
ArchDruid (CR 12) has 24d8
Warlord (CR 12) has 27d8
An archmage has 18d8+18 for 99 hp. This is exactly 4.5 per d8. An 18th-level wizard would have 92 assuming +1 CON bonus. A difference of 7 hp.
An archdruid has 24d8+24 for 132 hp, also 4.5 per d8. An 18th-level druid would have 117 hp assuming +1 CON bonus. A difference of 15 hp.
A warlord has 27d8+108 for 229 hp, or 4.48 per d8. An 18th-level fighter would have 184 hp assuming +4 CON bonus. A difference of 45 hp!
So, not only do they give more d8s to non-casters, but by comparison to the classes, they buff non-casters in HP even more.