Because the game mechanics are a representation of the fluff. This is a basic concept of game design, yet another one that you don't seem to understand.
2. Even if fluff did matter to game mechanics, how is it somehow "unfluffy" to have an army of cultists with a dark apostle as HQ?
Because it's Codex: Chaos Space Marines, not Codex: Cultists. I think it should be obvious why a fluffy chaos space marine army should actually have some chaos space marines in it. Fluff-wise the cultists exist so that your marines can have a wall of meatshields to support them, they're not supposed to be the core of your army. That's why there's a single cultist unit with very few options. If you were making a fluffy chaos cult army you'd probably do something like a core from the
IG codex (including a diverse range of infantry units) for the cult itself and maybe a small allied detachment of demons or summoning psykers (I forget if
IG psykers can take summoning spells) to represent the demonic forces they call upon. A
TAC list build like this would be just fine, and have nothing to do with a one-dimensional spam list from the
CSM codex.
You reject this argument when I make it it about SHVs and GMCs, as well as tank spam.
What gives you a pass to make it about cultist spam?
Because they have nothing to do with each other? Making the argument "X is not fun" does not mean I have an obligation to accept every other argument that something isn't fun. By your reasoning here I could turn around and ask you why you accept "
LoW aren't fun and shouldn't be in the game" but reject "tactical squads with missile launchers and flamers aren't fun and shouldn't be in the game".
You don't take advantage of all of the available tools. You don't use allies, as far as I'm aware: you ONLY use imperial guard. You don't even use all of the tools in your codex. You don't use rough riders, ratlings, ogryn/bullgryn, stormtroopers...do you?
You do understand that rough riders, ratlings, etc, are not in my army list, right? The only thing on your list that I have available and don't use is allies, and I don't think either of us want to make the assumption that a viable list has to use allies. Otherwise we're going to have to start asking why you think you deserve to win any games when you don't make use of the Eldar allies you have access to and take some Wraithknights of your own.
I didn't intend for a direct one-to-one correlation. This is the only point I'm making: in that scheme, you have A counters B counters C counters A. So if you want a "good" army you'd take a blend of the three.
Rock/paper/scissors is merely one approach to balance, and not one that is well suited to
40k. You can't assume that there is something wrong with balance if the game doesn't comply to your rock/paper/scissors concept.
You have an army of what should be fast attacks and heavy supports.
The fact that there isn't a target type where you look at it and say "nope, can't even try to counter that" is just bad game design.
And that "should be" is your personal opinion, not objective truth.
IG armored regiments exist (and, in fact, are pretty common) in the fluff and the army list for them is a fluffy representation of what they should be. Just like, say, Deathwing armies get to take terminators (normally elites) as troops an
IG armored company list gets to take
LRBTs as troops. This is balanced by their very limited access to the infantry units that make up the core of a codex
IG army. What you're doing here is getting tunnel vision on the idea that the unit allocation of C:
SM is the only way to do it, and anything that puts units into
FOC slots in different ways must somehow be bad game design.