I realize that you never played A16, but no, that's not the reason. The AI in A16 was in many ways profoundly stupid. Zombies basically just ran straight toward you from wherever they spawned. If you had a 5 block thick wall on one side of your base and a 1 block thick wall on the other side, if they spawned on the side of the 5 block thick wall, they'd try and tunnel through that. If you dug a 5 block wide trench all the way to bedrock all around your base, with bridges in each of the cardinal directions, unless you did something to slow them down, they would happily fling themselves into the trench (and even if you did something to slow them down, you'd still have some who would fling themselves into the trench.)
Heck, I once built a 5 block deep base with a 10 block long ramp coming down to me (completely uncovered) and what I ended up with was a group of zombies milling around above my head come morning. I'd even put in doors that I left open to try and lure the zombies down, and they didn't go for it.
This is a base I built in A16. That trench goes all the way down to bedrock (that's the whitish bit you can see on the near side, that's what bedrock used to look like.) I had zombies fling themselves into that trench and almost undermine my base. With the new AI, that wouldn't happen, they'd just happily cross the bridges.
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I mean, yes, I could have easily built a base that could withstand any horde (heck, like I mentioned you could just build an elevated base and stand there all night, since the zombies didn't try to attack things when they couldn't reach you, which was admittedly a problem, and why I mentioned adding the destroy area mode.) But I could also try all kinds of designs to see what worked. Now, like I said, the zombie AI is so predictable that I know whether a base is going to work (for the most part...I had one fail when they changed the length of path a zombie would follow) before I place the first block.
I built another base that was basically an octagon with tunnels to the center in each of the cardinal directions, the sections between the tunnels completely filled with reinforced cement blocks (hoping the zombies would "slide" along the angled blocks and end up in the tunnels) and they simply tried to dig their way through the walls (the zombies always came from NE/SE/NW/SW on horde night back in those days.) Nowadays, they'd have gone directly to the tunnels.
The point is, I could build these things and I would never know until horde night if they would work. Sometimes they worked. Sometimes they were spectacular failures. But I had to actually try them against the horde to see what would happen. I couldn't manipulate the AI to just walk into my death trap in an orderly fashion. And that's what I miss, the ability to try out ideas and see if they would work, not to be able to plan out a perfect base without issues.
If I could make myself forget how the AI works, that would be great, but I can't. Hence, boredom ensues.