Because you cant do anything sneaky, like go around, come from the sides and stuff like that... Can promise you that all mid would be 10 x better at the normal map.
The team with the longest/most ranged poke wins the game
You aren't supposed to be able to do that... that's the whole point of all random, all mid... Before they implemented the ARAM map, people kept it to only the middle lane without being able to do sneaky stuff like that. You can do that stuff in normal modes though
And ARAM isn't necessarily all poke. If you get a decent/good all-in team, then you can beat long range poke teams. There's also ABAM where you can blind pick whatever champions you want...