What League of Legends have you been playing?
move speed and knockback
stun and perma slow + ability that grants vision
traps and dash
blink
leap and knockback
bind and slow
slow
bind and slow
"blind" and knockback stun + ability that grants vision
spell shield and movespeed
blind, movespeed, perma slow that also grants vision
untargetable and stun
invisibility, dash, knockback that can stun
They have plenty of self peel on top of having range. Again, I don't know what League of Legends you've been playing, but it's been a very very very long time since I've seen any assassin kill a marksman in a team fight due to allies +
+
+

MARKSMAN ARE SQUISHY ONLY WHEN THEY'RE BY THEMSELVES! Mages are squishy even when their whole fucking team is around.
In my last game, I was playing 

and caught 
in mid lane so I dived them. Got Ashe very low, Morgana binded me, I hit my Q, Ashe flashed away. Q ended, I flashed then
ult'd me. Ashe had exactly 4 health! 4 FUCKING HEALTH! When the team fight started a few seconds later, guess who had 100% health again without recalling. I'll give you a hint, it wasn't

Why did Ashe get to heal herself back to 100% health with
, negate a lot of my damage with
, and even if I killed her, get a second chance at life with
. Which also reminds me, mages got screwed over on GA as well. Speaking of mages being screwed over, guess who the first to die in the team fight?
Good old Morgana. Morgana who blew her
so I couldn't execute her, giving her enough time for Warwick to come ult me. Morgana who could have healed a decent amount of health with her W on a minions and a few teammates and had a chance. But no, screw everybody that isn't a marksman.
excel in killing squishy targets in the chaos of a team fight. Most of the time going 1 for 1. AP assassins, AP marksmen, AP mages drop dead instantly unless they have their
(which still takes them out the fight for 2.5 seconds).
You know who doesn't die? The AD assassins and AD marksmen. Why... because they have
and
. If that isn't enough to sustain, they have
which takes them out the fight for 4 seconds and brings them back with 50% of their health, 30% mana and they still have their
healing on EVERY SINGLE AA coming in a 2AA/s minimal.
restores no health and no mage item grants healing. Again, mages get bent over and screwed by RIOT while ADCs get a step stool to stand on while receiving amazing fellatio from RIOT.