Because there's just no need for Item Clause. It's one of GF's more moronic rules. It also screws with sets that require a certain item that's already being used by another pokémon.
Plus, Showdown doesn't play by GF's rules to begin with.
I guess I don't see it as a "rule" though, but rather an actual mechanic of multiplayer battling. Like, Team Preview vs. banning an event Pokemon like Meloetta, for example.
Team Preview's mandatory whenever you battle against another player on an actual DS system, and there's no way around that.
Meloetta's restricted in random matches, but if you know the other player, then it's allowed. Whether Meloetta can be used or not is an optional setting, in that case.
I see Item Clause as closer to Team Preview than a Meloetta ban. It's just something that's part of the battling system for multiplayer in general, rather than an optional ruleset. So I'm not looking at it in terms of how it affects balance in the metagame, but more like how the simulator's not faithful to a fundamental mechanic of the battle system in general.
I've always found the lack of item clause on simulators rather silly, as there are plenty of items that perform similar functions such as the Toxic Sludge and Leftovers.
Although, yeah, on another note, I think it would also be fun and spur on more creativity and variety for teams if Item Clause were in effect.
the real question is why doesnt showdown use freeze clause?
i lost a battle to that nonsense
This too. I haven't lost to it this gen, but I literally had a match that went another 30+ turns because my last two Pokemon were frozen. Even Netbattle had Freeze Clause.