1) because they're used enough in OU to be in OU. In other words you'll see atleast 1 in 20 battles in OU which is a 3.41% but both are higher then thatWhy are landorus therian and garchomp OU? To me they have huge targets on them (one shot me with an ice move) and you never know which pokemon on their team has that one ice move no matter how rare that will one shot you easily, constantly living in fear of that ice move that will one shot and lose you the game, keldeo with icy wind, jolteon with hidden power ice, Victini with glaciate the fear is always there lol.
Ice in general seems really overpowered if i have 3 punches (fire, ice and thunder) and i predict you will switch out i will go with ice every time why? strongest status in the game, one freeze and the game is won very often, the variety of pokemon that ice either one shots (4x weakness, dragonite, gliscor, altariai) the popularity of dragons and the diversity of pokemon that are weak to ice all flying, dragon, ground and grass the types mentioned are some of the most common types in the game, ice is often the best move to use as a neutral hit aswell in hopes of that freeze. Am i wrong in thinking ice is just the best coverage move going?.
My last observation is that physical attackers are at a huge disadvantage in general so why would you bother even using them? for example Iron barbs, rough skin, rocky helmet, Burn, Intimidate, Spiky shield. All things only applying to physical attackers there is no such equivalent for special unless i'm mistaken so really what's the point in physical attackers?
2) ice is the best offensive typing indeed but you shouldn't rely on getting a freeze but if you get one yeah your odds at winning are suddenly way higher (depending which mon gets frozen ofc)
3) if you won't use any physical attacker, doesn't any random special wall win against you then? That's exactly why you run physical pokemon (plus beside prankster nature power and vacuum wave all priority is physical)