Why does ARAM use the same champion pool for both teams?

You never play against the same champion as someone on your team. Which means that any rerolled champion also is "used" by your team. This can heavily impact the little balance which can be found in an ARAM game. Re-rolled champions are usually those who are not fun/easy to play as in ARAM. So every Re-roll your team does means 1 less champion to hopefully faceoff against. I do not see much of an issue with facing the same champions because of a "random" drawing. If anything it adds to the randomness. So why both sides pull from the same pool is beyond me.