I know people don't want it, but why can Irelia 1v5 if everyone is at full build?

I just played a 3v5. And my team lost. And the enemies were feeding. Why did we lose? Well partially because my team's Lissandra sucked and didn't ult Irelia at good times. But that's not the point. Literally she could jump into our team and one shit 3 of us. And this is with a Gnar, Volibear, Lissandra, Soraka, and Yasuo. The amount of cc and damage we had wasn't enough to stop her with her passive (Which is a fine passive, it is just bullsh*t that she has so much Tenacity. I wish you couldn't buy Tenacity on her) and her attack speed and her damage. She had 3000 health. Against about 12000 health. Seems fair that she can deal 12000 damage and walk away with 1500 health left. Even after everyone ults her. So please, nerf Irelia's damage.

P.S. Yes this is a rage post. Congrats you guessed it. Want a medal? You'll get it with Champion Mastery on PBE have fun.