That is true, but I'd remind you that the current bestiary is incomplete. Besides, fighting 1st level NPC spellcasters is something that could easily happen in a campaign.
Yes, but they're deliberately avoiding changing the monster math & numbers. Easier to change 8 classes than 50 monsters.
I think the reason they made monster damage so low is because player HP are so low. Low-level monsters in 4e, for example, did more damage, at least as far as I recall. You may wonder why I'd want monster HP to go up, as wouldn't that just cancel the gain of more HP? Well, yes, it would. But it would also make damage more consistent.
Given monsters and PCs don't need to obey the same rules, I don't see why they need to have symmetrical damage.
Further, one of their big goals with bounded accuracy is that you should be able to use things like goblins as threats for more than just the first couple levels. But if they only do 2-4 damage on average, how is that a threat to, say, a 5th level character that has around 50 hp? This is also why I want player HP to decrease more slowly. HP bloat ruins many of the benefits of bounded accuracy.
Let's do the math.
Well, if you gain 5-8 hp per level that means an extra one to three goblins per level negates any extra hitpoints in a single round.
How are goblins a threat to a level 5 character? Well, a level 5 fighter with 14 Con will have 44hp (12+8+8+8+8). If he's in splint with a shield he's rocking AC 19, which is pretty high.
The rules say an Easy encounter for a L5 character is 150, an Average is 250, and a Hard is 500. A straight goblin is 10 xp. So an "easy" encounter is 15 goblins!
Still thinking that near-50hp is going to make him unkillable?
Goblins have a +4 to hit melee and a +5 range. Given there's 15 of the buggers, half will be attacking en mass from a distance. At range they need a 14 on the die to hit, so they're going to miss 65% of the time, hit 30% of the time, and crit 5% of the time. And at melee the numbers go down by a percentage.
Average damage is 1.6 and 0.8. It will take the fighter at least fifteen rounds to kill all the goblins, assuming he never misses (which he will given he still only has a +5-6 to attack and goblins have an AC of 14, but he'd also have multi-attack by that point so we'll call that even). Assuming the goblins are spread out so only a couple are in melee at any given time, the fighter takes over 20 damage the first round, is near dead in the second, and quite dead in the third. Of course, he's likely parrying so that might buy him a few extra rounds.
Now, in actual play, the fighter will hopefully be sticking to cover and will have friends blasting the goblins with AoEs so the tone changes dramatically. But a horde of goblins is quite an effective threat.