Why are Mages allowed to be better at assassination than Assassins?

Burst mages such as Syndra, Veigar, Brand and Lux can pop you like a grape from the safety of their backline (the latter can do so from like ten screens away). Clean, 100'd, Syndra doesn't even need to land a skillshot to do it. Control Mages can do the same, while also CCing the rest of your team and possessing an obnoxious amount of utility (AND OFTEN HEALTH) that makes them very safe. Orianna, Anivia, Viktor and etc. Meanwhile Assassin's who have to dive into melee range aren't allowed to 100-0 you because toxic, no counterplay, how is it fair that I die because I miss my CC and stepped out of position?!?

I mean people were always bitching about Rengar because 'dying to him was unfair, there's too small of a reaction window for it to feel like you can make a meaningful impact', but how is getting Instagibbed by a SUPPORT Brand camping in a bush, or a Syndra who's stacked her seven Dark Dragon balls of Death and flash-wished me dead any different? I mean if you hit me with the ward argument for Brand I'ma just clap back with the Pink Ward argument. Oh, and talk about interactive when Lux can just throw out Q's from over a screen away until one finally sticks, and when it does, you're dead. Meanwhile Nidalee, who functioned on a similar principle but had to at least dive in for the kill and possesses no CC and not as much utility was gutted. (Yes, I understand that the 'real reason' for her nerfs was because of how dominant she was in pro-play and very high elo, where she could exert obnoxious amounts of pressure. But lets be honest, the vast majority of people bitching about her fit into neither category, they just didn't like the idea of an assassin styled jungler being dominant.)

Then, to make it worse, they don't even need to be fed to do this shit. They just need to farm, or in the case of some cough Brand cough, not even that is necessary.