So let's start with our small skeleton army at level 5.
First, a 13 AC is not actually bad. A CR 17 monster has an AC of 19, so really AC isn't some huge thing that's only going to get bigger and bigger. Looking at CR 5 monsters, they range from 9 AC to 17 AC, typically around 14.
Additionally, 18 HP isn't that bad, either - although it's still hardly good. I had assumed that was fodder; in actuality, only one of the CR 5 monsters can do 18 damage in a hit - a hill giant, who can do it twice a round. Most CR 5 monsters do more like 14, which will typically take two hits. Now, every single one of these monsters can make at least two hits per round, but that's still an entire round wasted for one renewable skeleton.
Also, the +4 to hit (which never changes) isn't terrible. Granted: every CR 5 monster has about +7 or 8 to hit, but the average AC is still around 14, giving our skeletons about a 50/50 shot to hit.
However, a skeleton doing 8 damage on a hit is still pretty darn low. Most CR 5 monsters have 100 HP (84-126), and more importantly, resistance to non-magical weapons. (Amazingly, dragons don't have this.) So even 4 skeletons - our max at level 5 - will be doing an average of 16 damage if they all hit, and since they have about a 50/50 shot, that's about 8 damage per round, or 16 if the creature doesn't have resistance to non-magical weapons.
Now, that's still pretty amazing given that it is a bonus action, but keep in mind these 4 skeletons cost you one 3rd level spell (and a few days of prep time). Casting a 3rd level spell that does about 16 damage as a bonus action feels... about right, actually.
So let's fast-forward to level 16, given the example above. According to the online supplement, the Adult Blue has an AC of 19 and 225 HP. At this point our skeletons have 29 HP and attack at +4 for 5+5 = 10 damage. Attacking an AC 19 means we'll hit about 30% of the time, doing 10 damage, so we'll call that 3 average. 225 / 3 is 75, so you would need 75 skeletons to kill the Adult Blue in one round, on average. (I have ignored crits for this equation cause it just makes things harder; you can assume you'd need a few less thanks to crits but it's basically the same.) And as Jack said, this is within possibility for a 16th level necromancer (although my calculations say you can get as high as 82 skeletons with a 16th level wizard, all total, so it's basically almost 100% of your 3rd level and above spell slots).
HOWEVER, this is assuming that you were able to fit 75 skeletons into the space where the blue was, OR that it was dumb enough to let them all get within 80 feet (especially since its lightning attack's range is 90, AND it can use a legendary action after the first skeleton's turn to move another 40 feet away, causing the rest of the horde to attack with disadvantage and effectively making them only hit on crits).
It would also require days and days of prep time, not to mention all the other practical considerations, and basically all of the wizard's spell slots. If a party basically had an unaware threat they could prepare for days to defeat, I would be willing to bet 75 skeletons is just one of many ways they could figure out how to do it very efficiently.
That said: the bigger point is that you COULD do it, you could do it ALONE, and that your ability to do tons of damage with a jillion little guys is actually practical - all fair points. You're basically enabling the use of all your spell slots in one action, potentially. It reminds me of the "Leadership" issue in 3.5, where someone figured out that a character with a high enough Leadership score and CHA could get a few hundred followers to shoot arrows and take down any threat. Unsurprisingly, this also didn't happen much in actual games. 