Why isn't Jarvan's ultimate a skillshot?

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Seems like it'd be a lot more useful and have counterplay

Almost certainly because it's got negative impacts, and can be negated by half the champs basic abilities and/or a summoner spell.

I play a lot of Jarvan. You trap allies in with you, you box allies out, you get killed, you have trist, fizz, and lucian just lol at you. It's not that good.

how would it function as a skill shot unless it worked like leorics ult in hots

This would only buff J4 to be honest

So instead of needing an enemy champion to ult to you can just ult anywhere by clicking onto a spot.

It doesn't change the point and click champion thing but now he can ult over walls or to places he wouldn't have been able to prior to this.