Why do we need assassins with these ridiculous mages?

TL;DR: What is the point to play asssassins, when mages are so strong overall and are much safer?

So I just got a game where Vel'Koz 1 shotted my Corki (with Tri-Force and Maw of Malmortius, the first gives just a little bit of HP, but the second is supposed to make sure you survive the initial AP burst), without landing anything except for his ult. So he pressed R, burnt my 1,8k HP, completely ignored the shield, because, guess what, it doesn't protect from true dmg. Oh, and while doing that, he also did damage to everyone else on his way, so we can at least double that dmg, considering he went for me, and others managed to get out. 3,6k dmg on one skill. That is a mage, that is not an assassin. I would expect him to have to land at least 3 out of 4 skills to be able kill me, but no. Just press R, deal around 2k dmg to everyone on the way, most of it true dmg, because now you dont have to proc the passive before ulting. That's great isn't it?

Then we got Syndra. IDK if i need explaining, but she turned into something Riot has try to get rid of while reworking LB and Rengar - point click 1 shot. Does Syndra has to land every Q, W, or E, to 1 shot someone? Not really. Enough Spheres around, and that's a 1 shot. Oh, plus she has a potential 5 man stun. Who need's tanks to initiate, when you have that?

Let's talk about Viktor. Probably one of the most known mages with a one-shot potential. 2 of his skills (which are not his ultimate) make him deal damage 4 times, with damage that is worth 4 skills. Oh and don't forget to add the lich-bane proc. And rylai proc. And his ult. He has been toned down a bit, but he's still able to one-shot people.

Now we got to Ryze. Not as strong in terms of one shot, but still with an ability to do that. I'm not going into much of detail on it, I haven't played him for some time, but I have recently heard Imaqtpie saying something like: "It's just better to go Syndra, and 1shot everyone, if she's banned, go Ryze and 1shot pretty much everyone, if it's banned too, just go for some other mage and 1shot everyone."

Veigar with his ridiculous scaling on every single skill, Brand and Zyra that deal more dmg as supports than some midlaners and adc, Malzahar with his unstoppable shove (did you know he regens MP from his E, so he almost never runs out of it) and 1 shot potential 1v1, Anivia with her E that does ridiculous amount of dmg for a 2 skill combo, Annie, Lux, Orianna (these 3 actually need to land every single skill to oneshot, but hey, they have strong utility or/and range).

Mages are somewhat not mobile, that's true, but they have safer lane phase due to their range, they all can shove lane, every single one of abovementioned have hard CC, they deal AOE dmg at safe range, and they have a 1 shot potential on primary targets (such as ADC). Oh and most of them are building rylai now which makes it a lot harder to duel them as an ADC. So why do we need assassins in this game, when you can play a much safer picks with a better early game, and better lategame scaling? (I didnt say a word about Exhaust neutering assassing, but that is included in the range part.)

Edit: This topic is about mages being overtuned, not about ADC being one-shot, although this matter still concerns me.