The thing is, you cant have assassins with the burst and mobility they have just for being assassins, on top of reliability.
If Riot makes reliable, always able to kill targets assassins, no matter how far they are, what is the point of mages that arent AoE machines? Why could I ever pick a Malzahar, Syndra or any non AoE combo-wombo mages when they have less burst, less safety, less mobility and less survivability that an assassin.
A reliable assassin is simply design cancer. It cant exist and be balanced and healthy because it could outcrowd literally everyone that could even share even a part of role/niche.
Not only that, but assassins with mobility and superior laning phase like Zed and Ekko take away literally any agency on laning from their opponents. If they are assured a power spike no matter what AND limit the champions that can lane fairly against them due to the tools given to them, that means their opponents are fighting a lost battle and literally nothing they do in lane can stop the assassin from doing whatever they want. Outside of basically forgoing laning phase and trying to counterpick the assassin for post-laning fights, playing a 100% passive farm/push lane is the only way to go for MANY champions and players.
Not to mention Riot has LOVED to nerfwork or outright gut assassins/mobility counters.