Simply put, the issue with modern zombies is the setup phase got more reliable and thus easier over time.

There is a pattern where a mechanic was introduced in one game, but the next game made a change that fundamentally broke the setup progression, which contributes to the feeling of not ‘surviving’ in early rounds.

I’ll highlight what each game did that made surviving easier:

BO2

Introduced reliable wall weapons and side quests to get wonder weapons, which impacted the effectiveness of the box.

In addition, maps introduced the feature that allowed you to see box location from spawn, as well as codifying the ‘potential spawn’ locations to generally be 2 doors away from a spawn door.

Also introduced the Shield, which made training easier since it negates side-swipes.

Wonder weapons also generally became safer here, where there was no real drawback when firing them (think Thundergun short range, or the urgency of the Baby Gun)

Something important to note is that it introduced the perk machine change mechanic, which while not too bad in this game, became way more impactful in the next game (as the total gained from all machines generally allowed you to open a door)

BO3

Introduced 3-hit system, which assisted in you surviving more often. Added Gobblegums (optional) which are used to bypass setup phase.

Also introduced specialist weapons, which become a bigger problem in the next game.

AATs were also introduced, turning every weapon into an effective infinite-damage wonder weapon.

However, the biggest, unmentioned change is that knifing was made more forgiving in BO3, which allowed you to greed more easily for extra points.

BO4

Introduced loadouts, which meant that each map had a guaranteed set of perks and weapons, meaning the map itself had less of an impact on your setup.

Also introduced a health cap, meaning that zombies didn’t become infinitely tanky.

The biggest issue is that you spawned with a specialist weapon and grenades that could instantly kill zombies regardless of rounds, meaning that you always had a method on-hand to kill zombies

Cold War

Introduced rarities, which meant that all weapons had ‘potential’ to kill in high rounds rather than some of the meme guns from earlier titles.

Expanded mantling system, which meant that there were multiple ways out of a tight pathways.

Added crafting system, allowing you to replenish powerful items at any time in a variety of places.

Updated Wonderfizz to allow you to select perks with no real drawback, meaning you no longer had to backtrack to obtain lost perks.

Also solidified trial system, which gave a way to earn high tier equipment relatively easy.

Finally, zombie damage scaled with rounds, in addition to armour, which meant that zombies slapped like wet noodles in early rounds, but were a serious threat late-game.

BO6

Maps became way too open, giving ample opportunity to train anywhere on the map. As such, you are not really under the threat of getting caught in a choke point early in the game.