I think it's because aram player, if they play solo, most likely want to make big play instead of beeing helpfull to the team.
When there is a teemo/shaco/kahzix/... I always try to buy a vision item to make them useless (even if the remove of ap from Hextech Sweeper hurt a lot if I play a mage)
So, when we have 2 adc, I always ask, if I'm a mage, that one of them buy a Lightbringer. Most of the time they don't so I rush Hextech Sweeper.
It's the same when the other team is full mage and you have a fighter/tank on your team.
Even if all you need to win the game is him going tanky with aegis and stuff like that, there is a huge possibility he will go full ad to get some kills
Sometimes you can't force poeple to play the smart game, and in aram this fact is more true than in every other mode cause "it's just aram..."