Akali's ult is low cooldown with resets
Diana's ult is low cooldown with resets
Kassadin ult is VERY low cooldown
Fizz can get kills without his ult. If they Hourglass the shark they still have the damage amp on them so Fizz can still kill them easily
Evelynn is hardly an assassin
Katarina ult has resets, and she can kill without ult anyway, no energy to limit her ability spam either (she also has the best teamfighting of any assassin while Zed has among the worst)
Nocturne is an outdated assassin and these days functions more like a DPS bruiser than an assassin. Regardless, his ult offers much more utility than Zed's
Talon I will give you some credit, although his is more versatile in that it hits in AOE, can be used to escape, and isn't completely nullified by Hourglass
Furthermore, every ult you listed is "press R to do damage". Zed's ult has pathetic base damage and is dependent on him following up with more abilities (and the requisite energy) to pump up the damage and pray to god it doesn't get Hourglass/QSS/Shielded/Kayle ult etc.
I won't deny that Zed can be frustrating to deal with but when you look at every other assassin it's easy to see how much healthier he is for the game