Why did Wizkids make all the monsters in the monster manual and beyond having crappy armor classes. Even their monster generator in the DM's guide has even 30th level creations to only have a 17 AC.  All this while allowing Characters to be able to gain 24,25, and beyond.  To me this is just a death sentence to the monsters. Even the Ancient Dragons don't have much of a chance of surviving battles.  It seems to me they wanted to make it so the player characters have little to no chance of getting killed. Where is the challenge or fun in that.  I am just curious because I made a monster I call Color Flow dragon.  The dragon changes colors at the start of every round and takes the hit points for that type of dragon.  Some people say it is broken, but aren't the player characters often broken with all the magic things they can get plus all the ways they can be buffed?  I am just curious to what you guys will say and yes I have my big boy britches on.  Just keep it civil please.

WizKids have nothing to do with what armor class anything has. They just make the miniatures.

As to your question, the answer is that missing feels like crap, so the math aims to have players hit roughly 60% of the time.

You seem to have Wizkids (a subsidiary of NECO) confused with Wizards of the Coast (a subsidiary of Hasbro), as Hasbro does not make products under the Wizkids branding.

For the same reason that most bosses in video games will have large hitboxes and HP pools. Constantly hitting a target but then having a large HP pool is more engaging than frequently missing but needing fewer hits. Plus, they’d need large HP regardless to weather hits from save spells, so AC needs to be a little lower to avoid making attack rolls seem ineffective.

Fifth Edition put a fairly low cap on how high you can get a PC's attack bonus.  That required monsters to have a similarly low cap on their armor classes in order to prevent fights from turning into a case of everyone standing around unable to hit each other.

Part of it is to also keep attacks and forced saves in rough balance with each other too.

The AC for monsters comes from two design goals from 5e

1. Magic items and buffs weren't supposed to be a necessary part of your power -- prior editions had a design of "If you don't have a +X weapon by level Y, you're going to fall behind the curve of monster power and become ineffective" and they were trying to get rid of that. Thus, monster stats are based on not having magical bonuses.
2. The expectation was something like a 70% hit chance, regardless of level (it actually looks to be 65%)

If you look at the table in the DMG, it turns out that a character with a starting stat of 16, who raises their attack score to 18 at level 4 and 20 at level 8, and has no other bonuses, has a 65% chance of hitting an equal CR monster at every level but 9 (not sure why the level 9 anomaly), and CR below 1 is treated as 1, CR above 20 is treated as 20.

Unfortunately, assumption #1 is not realistic in actual play.