Complete noob - How important are runes for damage and tanking

I have noticed a pattern that I would like to better understand. In ARAMs when I go ADC or mage, even from the first waves of ARAM, I do not deal any damage to tanks. However, when I switch to Tank I see both mages and ADC absolutely melt me even from the first moments of the game.

When I start to scale with itemization, let's say as Ornn, I am mage with all the classical anti-tank items, I do not deal much damage to Ornn. Then next game, I select Ornn, build the same items as my opponents did, but then a 4 items Morgana deals 650 damage with Soul Shackle or a Rumble deals 800+ damage with one flamethrower.

How important are Runes? How do I feel at times I get absolutely melted and I cannot kill a tank with 250 health despite using 2 spells and my ulti, but when the roles are reserved a Morgana just spikes me for 1.5 k with her ulti.

Am I loosing my mind here or are Runes that important?