Trinity force and tanks

so lately I've noticed champions with Steamroller builds and , these champions build one damage item and then nothing but tank, but still somehow manage to outdamage EVERYONE on the opposing team, not saying its unfair but if someone can (politely) explain why they dish out so much damage even as tanks I would really appreciate it because I'm tired of losing to them

[slayer-jinx-catface]

{edit} I understand and are fighters not tanks, BUT in 7 out of 10 games I see them in they build TANK with their only damage item being the yet they still do more damage than anyone on their team or my team, I was asking how this is possible with only one damage item no I don't care about runes or masteries because while they do help they don't make champions stupid strong