assassins are the only class consistently good enough to kill ranged ADCs and mages, both in and out of laning. Bruisers can't deal with mages in laning. Bruisers can't deal with ADCs in laning. Tanks can't deal with mages in laning, tanks can't deal with ADCs in laning.
The truth is, ranged champions are entirely too safe in this game. Mages are long ranged and offer a plethora of utility from wave clear to hard cc to burst and it all comes from the safety of range. Some of them are even long range, some of them are even mobile. Some of them are azir. The same applies to ADCS, being a variety of mobile, long ranged and some with cc to infinitely kite. Then there's kalista.
Assassins are the only champions who can typically get in, kill them and get out and be a threat consistently enough for them to see play. People complain about assassins stacking tank items but the reason why is because it's gotten to the point where many of them can't even get in long enough to kill a backline without instantly dying or being kited into oblivion or hard cc'd. So they're the perfect class to abuse sticky, beefy, damage over time tank items and simply outlast carries because it's their only means to doing it.
It's a problem riot avoided for too long and they ended up just making it worst. Ranged champions are too good and too safe in this game. Majority of the items should have reduced effectiveness on them like rylais, frozen mallet, maw, etc but they don't. So with this in mind, there's no reason to pick melee champions that can't stack tank items long enough to stay alive to be a threat.
Zed and Fizz are some of the exceptions that can build full damage and people hate their kits but that's what's required of a melee champion to build damage and actually be a threat to ranged backlines because exhaust destroys melee champions, Mercs suck, hard cc is literally in every ranged champions kit now and to make matters worse, you have supports that can now build items and heal you while making you unable to die.
It's disgusting how ranged champions don't have their power curved in this game. And people can complain about zed, fizz and so on and I won't argue with you but just understand this: *that's what it takes for a melee champion to be consistently successful in a game that plays a noticeably ridiculous favoritism in terms of balance to ranged champions.*