system
A chat on subreddit /r/pcmasterrace
human
I hate the direction the game has taken after 4. As someone aptly put it, the game is pretty wide but also shallow. I kept going back to civ 4 for years. No where near the same with V and was done with VI even quicker.
gpt
I remember reading the dev team's programming blog (a little bit) while they were developing 5.

The main takeaways I had were:

1 - They were moving to a more multi-threaded model, where different things (animations, music, AI) happened on different cpu threads in parallel.  This was a huge re-write of every aspect of the game's code, but promised to make the game much more flexible, impressive, and less limited computationally in the future.

2 - Because so much time was spent building the new architecture of #1, many individual game components had to be rewritten to fit the new software architecture.  Some components, like the computer AI, were almost entirely dropped as a result.

3 - Civ 4 had absolutely fantastic AI (my opinion), but by default, it is really hard to have any idea what the enemy ai is doing with its military.  The _only_ real option is to build heavily into spies into border cities, which requires a lot of micromanagement, and is generally ineffective and costly, or found a religion and use it to spy, which is very hard.  As a result, most players don't bother with this.  As a result, it is often surprising, and appears random to a player, when an enemy army appears on their border, even though if you cheat to pay attention to what the computer is doing, the computer is actually spending a lot of time carefully managing and moving troops and weighing the risks of war.  Players referred to these as 'surprise doom stacks', and complained about it often to the Civ team, since it meant at any time an enemy ai could unexpectedly appear on your border with enough troops to end your civilization.

4 - As part of Civ 5, the team was moving from a square board to hexes, a positive.

5 - Since they had no AI, and were moving to hexes anyway, they decided to try to implement a 1 unit per hex rule, to "fix" the "doom stack" issue.

6 - Since hexes + 1 unit per hex + a militarily strategically incompetent AI meant that the computers were absolutely not a threat militarily, the team decided that for Civ 5, they would attempt to bend the game to be less militaristically inclined.  They ramped up massively the costs of conquering multiple cities as an artificial punishment for militaristic gameplay.  Nevermind that this makes the game less based in reality.

Civ 5 was overwhelming successful (The long time span between Civ 4 and 5 meant Civ 5 was many people's first civ game, plus the hugely popular reputation for the series, and the beautiful graphics, helped it be an instant hit), so they kept the formula for Civ 6.

All in all, though, the above list just reads like a long list of ways they neutered strategy in the game, in order to sidestep technical limitations in their engineering process.  All they really had to do to fix #3, is give players some sort of warning logic about enemy troop movements, like the ability to build a border guard or something.

OP says his dad gets overrun by the Civ 6 AI.  I'm sure the AI is much better now than at release, but I don't understand how that could be possible for anyone who's been playing since the Civ 2 days.  I retried Civ 5 a year or two after release, and managed to conquer an entire continent with 3 units on one of the harder difficulties, iirc.