The issue is the abundance of people willing to dodge in ARAM, so if someone see their ARAM comp is crap (or even 'only ok') for ARAM, theres a fair chance one of them will dodge. When a game does go through, it often means at least one team has a strong ARAM comp. Dodge penalties people might actually be more inclined to care about might help this.
Theres also the ARAM only accounts with only specific champs unlocked. Unlocking all champs for the mode would solve this, but it also has other issues for people who have vehement dislikes for more than a couple champions.
Related, those with relatively small champion pools are more likely to have their personal pools diminished from the other 9 player in the game, so just because someone has 20 available champions (including free ones) does not mean that they have a 1/20 chance of getting any particular champion, so that can make things feel less random too.