Why people refuse to itemize correctly against certain champions?

I've just had a game where my team's Vayne refused to buy a QSS even though there was a Malzahar and a Morgana on the enemy team. Both with very dangerous CCs (suppression and root, respectively). The former can only be removed by QSS. Even so, Vayne refused to buy QSS, with the excuse that "Malzahar wasn't killing her so QSS wasn't necessary". Here is the thing, he would always ult her during teamfights and she would be focused by the entire enemy team.

Last week I've also had a similar situation occur on one of my games. The enemy Swain was extremely fed. However, my team's Caitlyn refused to buy a Executioner's Calling because Swain was supposedly "not carrying" (I don't even know what "carrying" means at this point since people use this word at random situations with random meanings) - However, Swain was dealing a lot of damage to our team and was unkillable because his ult's healing was insane. I was forced to buy an Executioner's Calling even though I was using Nautilus. However at this point, it was too late, and I've been forced to sell this item later due to slot efficiency.

I would like to know why people think that if a champion is "not carrying" (whatever this word means) this champion is not a threat. A good example of this are tanks, they may not be carrying by themselves, but with their CC and tankyness they are a threat because they can disrupt a team. However, as I explained above, tanks are not the only ones who can be a threat even if they are not ahead. However, in both of those situations, we could have an advantage if my teammates had itemized correctly.