When it comes to 40k, it's because 40k is very unintuitive system, and many armies cannot list-tailor to the same degree as others. Specialist armies (like Eldar), or armies full of weapon options and counters (like IG or Marines), can list-tailor to a far greater degree than other armies who are more limited in their selection process. The end result is even if you put it out on the table that this is a 'list-tailoring' game where you bring the armies designed to mercilessly crush each other, certain armies still come out ahead... There's also the fact that list tailoring ability of some armies have no bearing whatsoever on the opponent, but rather relies entirely on hoping the opponent didn't plan on their specific build-type (i.e. Orks).
In other game systems, list-tailoring isn't as much of an issue because the different factions are on a much more equal standing with one another. Or the game itself is built more heavily around combined-arms and overall strategy. But in 40k, neither of those are the case. The fact of the matter is if you list-tailor in 40k, then the game is bound to be completely unenjoyable for someone.