Er... because she can only do her damage if you literally all leap headfirst into her and LET her?
Illaoi is pretty crap design, to be fair. Her kit is almost ENTIRELY "punish people for trying to all-in". This makes her REALLY awkward to balance. There are only three things to happen when you're vs Illaoi. Either your team can avoid going all-in on her and she's pretty useless... or your team is melee-heavy and needs to go in to accomplish anything, in which case she's basically just a stat check. Either she has the items/raw numbers to win with ease, or she doesn't and she STILL didn't matter.
Interesting champ... poor gameplay.