Can someone explain to me why people build runaans on adc first in aram??

They're bad.

Well, since champions in aram are always grouped I guess that it is much easier to get that extra damage off on champion, and the attack speed is good of course

agree

Depends on the adc, certain ones might be able to justify it. If they are doing that on Varus or Graves then they're stupid, on Kalista I can accept it.

Usually because they can get most of the item early/at the start and finish it faster then BT or IE cause of the BF sword big cost.

could work on someone like Kog or MF who have deacent dammage thats on hit built into their kits

It's a recommended "essential" item for several champs, if I'm remembering correctly. People are probably just following those builds without really thinking about it.

They never played ADC, and they follow the recommended.

In ARAM, it's both harder to afford the big AD items like BT or IE and it's harder to scale up your damage into the later stages of the game with crit chance items due to the big AD items being harder to get. Runaan's Hurricane might just be a compromise solution that is both more affordable than the AD items and will have more impact early on than the crit chance items. That's just a theory, but if that's actually the case then Runaan's Hurricane actually makes sense as an early item. It's still a bad item, but in the context of ARAM it might just be good enough that it's playable.

At the same time, it's ARAM. You aren't here to have a competitive game, you are here to have a fun game. What's stopping you from trying out a fun item like Runaan's Hurricane in ARAM?